Wednesday, January 30, 2008

2. Nikon TTL Flash Metering System



Conditions: Flash in iTTL Mode - not iTTL-BL Mode.

(TTL means iTTL in this blog)

It is very important to remember that the Nikon TTL Flash metering system is not actively coupled to the Camera metering system. Even though these two systems use the same metering sensor in the camera, they act independently. This is definitely not intuitive and not explained well in any documentation that I've read. It is alluded to in the Nikon CLS pamphlet when it talks about using the FV Lock button to meter a subject that will be off-center when the background is dark.

Note: The TTL-BL flash metering system is actively coupled to the camera metering system, and I will talk about that in my next post.

The picture above was taken in nearly total darkness in camera Manual mode with the shutter at 1/8th second and the aperture at f/2.8. Notice how the flash system handled the entire exposure of the subject, while the camera system handled the the exposure of the background. You know this because of the shift in white balance from the subject to the background. Notice that the background is very yellow and the subject color is much whiter. This is because the flash was the only contributor to the exposure of the subject, and the background was lit by incandescent lights. The flash was so weakened by the time it reached the background that it did not contribute to the background at all.

The Camera metering system does not measure the amount of regular TTL flash that will be added to the exposure of the subject. On the D200 and all cameras prior to the D300/D3, the camera does nothing to help you with this. If you are shooting regular TTL flash in bright ambient light, you risk severe overexposure if you do not reduce the camera exposure when you turn on the flash.

On a D200 camera you can easily demonstrate this problem by taking a picture, with and without flash, with the shutter set at a fixed amount. Set the camera on S mode and the shutter to 1/80th and let the camera pick the f/stop. The f/ stop that the camera selects is the same whether the flash is on or off. This means that whatever ambient light is reflected from the subject will be further illuminated by the the flash, and if the ambient is already enough to properly expose the subject, the flash will cause overexposure.

However, on the D300 and D3 and newer cameras, Nikon has made a significant improvement to help with this. While the camera metering system still does not meter the amount of TTL flash that will be added to the exposure of the subject (like it does in TTL-BL mode), the newer cameras automatically reduce the camera exposure in bright ambient light when the flash is turned ON. For instance, in the camera A mode, in bright ambient light, the shutter speed is increased when the flash is turned on. The amount of the increase is dependent on how bright the ambient light is. This process is similar to the process that the photographer was required to do with flash compensation prior to the D300/D3.

The Flash metering system just fires the preflashes and looks for the reflected light in the center weighted frame regardless of which camera metering system you are using or what the camera f/ stop and shutter is set to. The flash metering area is affected most by centered subjects and the metering sensitivity decreases as the subject is placed farther and farther towards the edge of the frame. At the edge of the frame, the subject has much less effect on the flash metering. Whatever light reflects back from the preflashes in this center-weighted fashion is what determines the power of the flash. You can see that if the subject will be way off-center, your flash power will likely end up way too high as the flash attempts to light the background in the center. This is why the FV Lock function was invented, which I have written about here: http://nikonclspracticalguide.blogspot.com/2008/01/4-so-what-is-flash-value-lock.html .

So, you can reason that the ambient light reflected from the subject is essentially added to the exposure once by the Camera metering system and once by the Flash metering system and overexposure is often the result if the ambient light is strong.

Here is an example of overexposure with flash when ambient is strong.



One important operational consideration that should be drawn from this is that when the ambient light is bright, it is usually best to turn down the flash using flash compensation to about -1.7 ev to start with. This will usually avoid overexposure on especially the subject's face.

Note: This is what the newer cameras (D300/D3 and newer) do for you automatically, except they do it by reducing the camera settings, which reduces the ambient portion of the exposure. The thing that is not so nice about this is that if you use regular TTL outdoors in bright daylight for fill, this automatic reduction in camera exposure will also make the backgrounds darker. So this improvement is really only a band aid that protects the unknowing photographer from severe overexposure. The best way to shoot fill using regular TTL is to use the camera in Manual mode where you have full control of the shutter and aperture. Then, you reduce the flash power with the flash compensation as I mentioned above.

This is why Nikon invented TTL-BL, which I'll talk about in my next post.

176 comments:

Justis Photography said...

I keep finding myself reading these posts over & over. Nikon did do a cruddy job in the flash manuals but I have managed doing it the Lance way. I am so glad that you posts this stuff cause in the heat of the moment it is SUPER easy to forget and I am trying to think more academic about my flash instead of instinctive. I know seasoned vets who have no clue about some of the stuff your writing. I am talking people with at least 10 years wedding experience.

Anonymous said...

Wow...excellent content. First time reader here...and I wish I had read this before a bridal shoot a week ago. It would have saved me a lot of photoshop time (shooting in RAW saved my bacon). I only shoot Nikon and wading through their instruction guides is a laborious and sometimes unfulfilling task. Thank you!!!

Russ MacDonald said...

To Anonimous,

Great feedback!

Thanks,

Russ

Kane said...

Now I understand why outdoor a sunny day, my pictures are all way too bright. Thanks to this knowledge sharing, I am really excited to try out in the field! Thanks a lot!

Anonymous said...

Thanks so much for putting so much time into explaining this ... I'm just now getting my flash off camera and I'm so excited about having this information to reference!
Kasandra

Russ MacDonald said...

Hi Kasandra,

I'm glad you like my writing! Nikon's CLS is a great invention.

However, you need to be aware that most of what I have written thus far, is about CLS with the flash on the camera.

When you move the flash off-camera many of the things I have written about change, and especially, you lose TTL-BL, one of the most powerful features of CLS.

I plan to write more about off-camera flash in the future, but it is very important to understand on-camera CLS before you move off-camera.

Russ

sir_siano said...

when on ttl and ttl-bl modes, do you usually meter the exposure without a stofen or diffuser on the flash? what if a diffuser is on the flash head?

i noted that the the flash is always pointed forward in your examples. would the metering still work if the flash was bounced off the ceiling or walls towards the subject.

thanks.

Russ MacDonald said...

sir_siano,

TTL and TTL-BL modes work with the flash in any position and with any diffusers or other modifiers in place.

The system works by firing preflash monitoring pulses and looking at the reflected energy. Those preflash pulses go through all the same modifiers as the main flash, so the system adjusts for any modifiers you have installed.

However, the flash will always have to fire at a higher power (which it does automatically) when you have modifiers installed, so the maximum flash range is decreased, but there is no need for changing anything for the metering process.

Also, my flash is almost never pointed straight forward. For the pictures in this particular post, I was using a Gary Fong Light Sphere II on my SB800 flash with the flash pointed straight up.

Russ

sir_siano said...

Thanks, Russ, for the clarification. I was under the impression that your flash was always pointed forward when you took those pictures.

Now, if i could only find the reasons why my pictures almost always turn up underexposed (ttl indoors with my sb 800 on my nikon d50)even if i follow the instructions in your articles. maybe it has something to do with the exposure compensation.

Russ MacDonald said...

sir_siano,

Getting consistent flash shots takes a lot of practice. I am always analyzing the scene to try to figure out what the flash computer will do to the flash power. The flash will fire at a lower power when the subject is light colored (like a white dress), and it will increase its power when the subject is dark (like when wearing a dark suit). A high quality DSLR camera will always tend to protect the highlights which will leave an image dark rather than blow it out. Point & shoot camera often blow out images as they try to do full brightening in-camera. That's why faces are often 'washed out' with P&S cameras.

If you are shooting raw, then you should always leave your pictures a little dark and brighten them in post processing. You have lots of room for adjustment in raw.

If you want finished jpegs directly from your camera, then you will need to brighten them in the camera. However, doing this without blowing out faces is very difficult. You have to check the histogram on nearly every shot. Face pixels should never be pushed all the way to the right edge on the the histogram, and Caucasion faces are often the brightest pixels in indoor flash images, so they tent to be the pixels on the right side. The Nikon Active D-Lighting goes a long way toward making finished jpegs in-camera.

Anyway, if you are always getting dark pictures when shooting TTL indoors (ie, when flash is primary), that means the flash is not firing at a high enough power. There are several ways to increase flash power. I normally use the flash compensation on the back of the flash. Adjusting camera ev will also change the flash power, but I normally never use it becasue it also affects the ambient portion of the exposure.

Whenever you increase flash power, you always need extra caution to watch the histogram very carefully to avoid blowing out faces.

It is far better to have an dark image that you can brighten in PP than a blown out one that can't be fixed.

Russ

sir_siano said...

Again, thank you for your prompt response.

Yes, practice. There is no quick fix to what i'm experiencing but to practice more.

I have lately been fascinated with using my sb800 even outdoors in broad daylight to give my subject/s that pop. And what i do is meter for the ambient background or sky, underexposed it by 1 - 2 f/stops and shoot using ttl-bl or manual flash settings. Sometimes, the image turn out ok. It underexposed, i adjust the shutter speed accordingly.

I have never given histograms that much importance. After reading some articles, however, i now understand how images taken in raw can be greatly improved thru photoshop (levels) and the use of the left, middle and right sliders, whether to add more shadows or highlights. And how by looking at the histogram after i take a picture can i greatly improve the image by a re- shoot at different exposure settings.

Again, thank you.

Russ MacDonald said...

sir_siano,

Outside when shooting fill flash, you should expose the background normally. Don't underexpose it.

The whole idea for fill flash is not to make the subject 'pop', but to add a slight touch of light on the face to lift the shadows. The main goal is to make the image look like it was shot without flash at all.

If you underexpose the background and increase flash to relace that light on the subject, it makes the subject stick out unnaturally from the background. That is called shootiong 'hot' and that effect might be OK in some cases, but most of the time you want the outdoor background bright like it naturally is.

When I shoot fill flash in bright light, I use S or P mode on the camera (to avoid running into max flash shutter speed of 1/250th), TTL-BL mode on the flash, and I normally DECREASE the flash ec to -0.7 ev. This makes the image look natural and as if it had been taken using only available light. The difference from the real available light is that you can see all the details on the face.

Now, indoors, where you want the flash to be primary, it is a different story. Then, you use the technique you described, except you should use regular TTL, or your flash will turn itself way down and you images will be dark.

Whenever you want the flash to be primary on the subject, you should use regular TTL (even outdoors in bright light, but only if you want the flash primary). When you want the flash to only add fill, use TTL-BL.

Have you read all my blog posts on this?

Russ

Russ MacDonald said...

sir_siano,

A little more about histograms and the Levels adjustment:

The most important slider in the Levels adjustment is the righthand one. Moving it to the left will brighten your pictures greatly.

If you have a dark image, the pixels will be gathered in a hump in the middle of the histogram. Just move the righthand slider to the left until it reaches the edge of that pixel hump. You'll be amazed at how much that improves your image.

Then, move the lefthand slider to the right until it gets to some pixels. This darkens the dark areas and increases contrast in the image.

I rarely move the center slider unless I need even more brightening than the righthand slider will give.

In the camera, you can achieve the brightest image by increasing the exposure (increase flash power) to push the pixels to the right edge. This is called 'exposing to the right', sometimes abbreviated ETTR. But don't push pixels so far right that they begin to climb the righthand wall. That will cause blown highlights.

Then, check the blinking highlights screen to make sure you haven't blown out anything important. It's OK to blow out specular highlights, but never faces or clothing.

Normally you should not ETTR when a face is the brightest portion of the image. Face pixels generally look best when they are positioned at about the middle of the 4th quartile on the histogram.

Russ

sir_siano said...

Again, thank you for promptly responding to my queries. I really appreciate the lengthy explanations on the settings one can use when shooting with flash in bright sunlight and the use of histograms in the levels adjustment in photoshop.

sir_siano said...

A friend would like to know if there is an override to having the diffuser on SB800, and still keep it in auto zoom? We know that if the diffuser is installed, zoom is fixed to 14mm.

Russ MacDonald said...

sir_siano,

The question is, why would anyone want to override the operation and force the flash to zoom with a diffuser in place? The whole point of the diffuser is to scatter the light as much as possible, and having the flash in its widest mode helps achive this. If you made it zoom with a diffuser attached, it would just counteract the diffuser and make it less effective.

Remember, a diffuser works best when pointed straight up or at a wall to get as much bounce from dofferent directions as possible. You get no softening at all when you point a flash directly at someone outdoors with a diffuser installed. Indoors, the softening is totally due to the bounce and you want to always scatter the light as much as possible to get the best bounce.

Russ

sir_siano said...

I understand what you're saying about the role of diffusers. What i cannot understand is why some photographers slit their stofen so that it wouldn't depress the small pin on the underside of the flash head (the small thingy that fixes the zoom at 14mm) so that the autozoom function is maintained. Is there a rationale to what they're doing?

Russ MacDonald said...

sir_sianno,

I've been a professional photographer for the last seven years, and during that time I have been told all sorts of wrong information by others with far more experience than I. Even my boss at the studio, who has 30 years ot professional experience didn't understand many of the finer details of the Nikon TTL system, and his photos, while good, could have been better.

I see no possible reason to defeat the wide angle provision when a diffuser is attached unless someone is in a studio and wants to throw some diffused light on a particular spot. Possibly you could use this idea to form a hairlight or a background light. I use other meathods to accomplish this (snoots, flags, grids, etc) that work so well that I don't see any reason to try anything else.

Russ

billmoris said...

Hi, my confusion start when metering mode gets into the mix. I understand that when using the pop up flash, Matrix and center weight acts as a fill (ttl-bl) and with spot it's not (just tll). So, when I take a photo inside with low ambient with the pop up, should I use spot metering only? You mention that we should not us balance flash inside right? Thanks.

Russ MacDonald said...

Hi Bill,

Your question is a very good one.

Yes, indoors, in low ambient conditions, you will get better results if you use spot metering. It forces the flash into regular TTL which lights the subject to a 'standard' brightness without regard for the background. Then, you set the brightness of the background with the camera f/ stop and shutter speed.

Have you read my other post here?

http://nikonclspracticalguide.blogspot.com/2008/01/nikon-flash-two-separate-metering.html

Please read all my blogs on this subject and I think things will be much clearer.

Russ

billmoris said...

Yes, I have been read the blogs, but I need to read it a couple of time more to absorb everything in. However, I get a little more confuse when another factor is added, like the metering modes with flash and off camera.

You said in this post that Camera metering don't take account for ttl flash. So if I meter my subject in manual mode with the buit-in lightmeter to center, and then use flash either pop up or a speed light, then my photos will be over expose all the time, correct? To correct this, I have to decrease my flash compensation or don't use the lightmeter with flash.

Russ MacDonald said...

Bill,

You are exactly right!

When you shoot in TTL mode, the flash thinks it is the only light on the subject. That's why you should always meter a couple of stops below the ambient when shooting TTL, so that your camera will not add much brightness to the subject. Then, TTL does a fine job of controling the brightness of the subject.

If the ambient is bright, and you can't adjust the camera to remove the ambient portion of the exposure from the subject, then you have to turn the flash down just as you say. In that situation, I normally start with the flash at -1.0 ev.

Now, this is normally not an issue. If you are shooting in normal indoor lighting, the best way to handle it is to use camera Manual mode, at 1/80th, f/3.5, and ISO 400 and leave it. That will be about two stops underexposed from the ambient which will allow the background to show dimly. Then, the flash will automatically give you a proper exposure of the subject - nice and bright - two stops brighter than the background.

Now, outdoors, where the ambient is so bright you cannot make the flash the primary on the subject, you have to switch to TTL-BL and let the flash provide fill to lift the shadows. Then the camera metering does affect the power of the flash. However, if you set your camera so that the meter is not centered, the flash doesn't know about that. The flash assumes you will center the meter.

The flash power also responds to the camera ev settings.

I hope things are getting clearer.

Russ

billmoris said...

Thank you Russ for the prompt replies and in-site. It's getting clearer little by little.

I hope I am not asking too much questions here, but I am sure there are a lot of us out there who has these same questions, and who would benefit from some reiteration.

So about meter mode. Can you clarify how different metering mode effect ttl and ttl-bl flash?


We know that Matrix takes in account the background and balance it with the subject, center weight takes the center more, (both acts as ttl-bl so I will consider them one in the same) as oppose to spot which is in ttl standard.

1.

So when using a speed light with ttl, none of the metering mode matters, right? It doesn't effect subject's exposure because the camera meter mode is different from the flash metering mode. Only the shutter speed with effect the background, right?
Does the ttl of the flash override the matrix and CW balancing effect, right?

2.

Likewise, when using as a ttl-bl as a fill, how would metering mode effect the exposure?
If I remember correctly, you said it does have an effect. In this case, matrix and CW will increase exposure respectively. Spot will not consider the background at all and will be have the most exposure (brightness)..


3.

The last factor is off camera flash which works only in TTl mode.
Is this the same case as on camera ttl in regard to meter mode?

4.

And lastly, will meter mode effect exposure when you take ttl out of the picture and use manual flash?


Again I feel bad asking so much, but the questions are stressing me out. I just wanted to clarify a few things. May be this will be a good blog topic. Thanks again Russ.

Russ MacDonald said...

Hi again Bill,

No problem with all the questions. Like you say, others may be reading this exchange and learning as well.

First, it's not Matrix metering that takes into account the background. It's the TTL-BL mode on the flash that does that.

Also, all my coments are for an external flash in the hot shoe - not the pop-up flash. The pop-up flash works slightly differently.

Also, the term background brightness means the same thing as ambient brightness, so don't be confused if those two words are interchanged randomly.

Matrix metering is a camera metering mode that simply reads the entire frame and that information is used for two things. 1) setting the f/ stop and shutter on the camera to control the ambient (background) exposure, and 2) it is sent to the flash computer as a proxy for the ambient (background) brightness for use in TTL-BL mode.

Then, there is a second metering system: the flash metering system. This system works by the flash first firing a preflash pulse and then measuring the reflected energy from the center part of the frame (where the subject is). This tells the flash computer how bright the subject is. This is also why the subject needs to be in the center of the frame for good TTL balancing.

Then, in TTL-BL mode, the flash computer takes that ambient (background) brightness from the matrix metering (or CW metering) and compares it to the results from the reflected energy it measured from its preflash, and sets the flash power to make the subject brightness equal to the ambient (background) brightness. This is the balancing that takes place.

Now, if you use camera CW metering, and you are in TTL-BL mode, that data will also be sent to the flash and the flash will use that as the proxy for the ambient (background) brightness. The problem is that CW measures only the center part of the frame and that doesn't do as good a job as Matrix in providing an accurate representation for the background brightness. This means that in CW metering mode, you won't always get as good a balance with your flash as you do when using Matrix mode.

If you select Spot metering mode, the camera meters only a tiny spot in the center of the frame that is assumed to be placed on the subject. In other words the data from Spot metering contains no information at all about the background brightness; it only contains information about the brightness of the subject. Maybe you can see that in Spot mode that there is no ambient information for the flash to balance with. That's why when you go to spot mode, you are not allowed to use TTL-BL on the flash.

All metering modes always affect the ambient part of the exposure. Only the matrix and CW modes are allowed with TTL-BL. In TTL-BL mode the camera metering tells the flash how bright the background is, so those two modes also affect the flash power, since the flash is adjusting to balance the background brightness with the subject brightness.

It's important to understand that a flash picture always is made up from two parts: ambient and flash.

When you want the flash to overpower the ambient, like indoors in dim light, you use TTL mode, and any camera metering mode you want. I normally use Matrix. In TTL mode the camera metering has no effect on the flash, so you get the same amount of flash no matter whether you use matrix, CW, or spot metering.

When you want the flash to balance with the ambient, like outdoors in bright daylight, you must use matrix or CW mode (matrix preferred) on the camera and select TTL-BL mode on the flash.

When using the flash off-camera, TTL is the only TTL mode available; TTL-BL is not available. Again, when using TTL, on or off camera, you can use any metering mode you want to.

Remember that the pop-up flash is restricted to spot mode for TTL. I have been only discussing an external flash in the hot shoe.

Hopefully this is making more sense.

Russ

billmoris said...

Thanks Russ, this Blog is Gold.
It should be printed inside the the Nikon flash manual, with your credits on it.

I think you cleared up almost all confusion. I now understand TTL and TTL-bl with hot shoe flash and off camera with your excellent detail explanation.

Two question remain though...sorry.

First is, what do you mean pop up flash works differently? In regards to metering mode.

The second is just a clarification, that with manual flash (no TTL mode, commander mode set to manual or when using a strobe light), meter modes does not matter at all also.

Thanks again and again.

Bill

billmoris said...

Ah, I see you have already answer my first question in Topic 8. Thanks.

Russ MacDonald said...

Hi Bill,

You wrote:
"what do you mean pop up flash works differently? In regards to metering mode."

Well, this is not documented well by Nikon anywhere.

Nikon tells us that the pop-up flash always stays in TTL-BL mode if you have Matrix metering set on the camera. However it appears to work correctly indoors when the flash is primary. Nikon documentation just recommends leaving the camera in matrix metering mode even when shooting indoors. This indicates the flash somehow knows to switch itself to TTL mode to avoid dark images,
but Nikon doesn't say anything about this in any of their documentation.

So, a beginning photographer can just leave the camera in Matrix metering while using the pop-up flash indoors or outdoors, and the images are good.

As I have discussed, this is not the way it works with the external flash in the hot shoe.

Russ

sir_siano said...

Russ,

I don't know if this is the appropriate forum but i really need your help.

I take pictures of dance recitals (street and cheer dance)and i almost always end up with either dark, under and overexposed, blurry pictures.

If the use of flash is allowed, could you please recommend appropriate camera settings which i can adopt so my pictures wouldn't end up bad.

I'm using a nikon d50 and a nikon sb800. The lighting at the venue is always changing. I would be about 15-20ft from the stage and i use a tamron 28-75mm f/2.8 lens.

Should i go manual or program mode? TTL or manual in the flash? Use a stofen or not? Auto ISO or not?

I know i'm asking a lot but i've been taking pics of events (dances and concerts) for so long that i feel i haven't been learning from previous shoots though i've been applying what i read from your articles and others. Help please.

Russ MacDonald said...

sir_siano,

Please give me your email address and I'll help you outside this blog.

Regards,

Russ

Anonymous said...

Dear Russ, thank you so much for all your posts; they are very informative and easy to read!

I am still a little confused regarding the statement "the camera metering system does not take into account that the TTL flash will add to the exposure of the subject." I noticed that the SB800 is able to read the camera's ISO, focal length, and aperture settings, so what is the point of the flash doing this if the camera will not take into consideration the TTL flash power?

Also, there must be some communication between the flash (in TTL mode) and camera (in matrix or center weight mode) because when I first took a picture without flash inside my house, with only a small lamp on, my camera indicated (in P mode, ISO 200)a shutter of 1/2 sec, aperature f/2.8. Then I turned on the flash in TTL mode, and now the camera indicates an exposure of 1/60 sec, f/5. Please clarify!

Thank you so much for your help!

Russ MacDonald said...

To Anonymous,

The ISO, aperture, and focal length are sent to the flash so that it can figure out its maximum distance and display it on the back of the flash. The flash doesn't use this information for determining flash power. It is only informational information that is displayed for your use on the LCD of the flash.

In fact, the flash computer is actually in the camera, and that is where the flash power is calculated for TTL and TTL-BL modes.

The camera metering system measures the ambient light of the scene. This information is used by the camera to set the camera aperture and shutter based on the camera mode. This ambient metering data is not used by the flash computer in TTL mode, but it is used for TTL-BL mode.

When in TTL mode, the system assumes that you want the flash to be the primary illumination on the subject, so it does not use the ambient metering data. It simply looks at the strength of the reflection of the center-weighted monitor preflash, and it assumes this reflection comes only from the subject (which means for this to work well, the subject needs to be in the center of the frame, fill a reasonably large portion of the frame, and be significantly closer than the backgropund). Then, it sets the flash power as if the subject were in dim room; ie, the flash will be doing all of the illumination on the subject.

This means that if there is significant ambient light on the subject, this will add to the flash and the subject can end up overexposed. This is why you should not use TTL in bright ambient conditions.

And this is why TTL-BL was invented. TTL-BL is the most advanced mode there is.

In TTL-BL mode, the flash computer uses the ambient metering data from the camera, plus either the reflected monitor preflash strength (if the flash head is not pointed forward) or the focal length (if the flash head is pointed straight forward), to set flash power so it will light the subject to be equal brightness to the ambient.

Your second question relates to what happens when the flash is turned on and the camera is in P mode.

There are different 'Programs' that are run by the camera computer based on which mode the camera is in and whether or not the flash is turned on.

In P mode without the flash, the camera calculates the settings for the aperture and shutter based only on the ambient metering data.

When the flash is turned on, two key things happen.

1) The camera now uses the 'Flash Shutter Speed' that you have chosen on the menus as the minimum shutter speed. This is the 1/60th default you are seeing. This speed is chosen because this is the lowest shutter speed that will avoid background motion blur from handholding the camera. If you go into brighter ambient light this shutter speed will increase as necessary to control the background brightness.

2) The aperture is set based on the lens that you are using. The P mode program chooses what it feels is the best compromise between flash distance and depth of field. Indoors in dim ambient light, it will typically choose an f/ stop that is not wide open on your lens, but it is close.

So, the result is that when using flaqs indoors in dim ambient light the P mode (and the other auto modes) always leaves the background significantly underexposed, and the flash lights the subject to correct brightness. This is one reason why most people use camera M mode when shooting indoors, so they can choose the aperture and shutter to give the best background exposure.

I hope this helps,

Russ

Anonymous said...

Dear Russ, thank you so much for your quick and very informative reply! I now understand what is going on with the flash TTL and TTL-BL modes. Thank you again!

-Lilian aka Anonymous :)

Anonymous said...

Russ, I have another question for you. Recently I took pictures of my friends at a wedding. We were on the dance floor, so very low ambient lighting. I think my settings were ISO 400, aperture priority, flash in TTL mode. When I used the flash pointing straight at my friends (4-5 ft away), they were overexposed. I had to point the flash towards the ceiling and use the bounce card to get a "proper" exposure of my friends. I cannot figure out why this is...

Thanks,
Lilian

Russ MacDonald said...

Hi Lilian,

TTL flash tries to expose the subject all the way to the right edge of the histogram. Then, based on the color of the subject (skin color, color of clothing, etc) the exposure might be high or low.

You can decrease the flash power with the compensation on the flash.

Highly reflective colors cause the flash power to decrease (especially white). Dark colors cause the flash power to increase. This can often cause overexposure of the face.

Also, if you subject is not centered, the flash will try to light the background, and this can cause overexposure of the face.

When I shoot flash indoors, I use -0.3 to -0.7 ev on the flash as my normal starting point. This will minimize the number of overexposed images. However, that also means you have to brighten most of the images in post processing.

Also, you will never get very good looking images when you point the flash directly at your subject. It is almost always better to use a diffuser and point tne flash straight up letting the light bounce mostly off the ceiling..

Russ

Anonymous said...

Russ, thank you so much for the info! I looked over my pictures and realized that most of my friends were wearing black and dark colors; no wonder their faces were washed out!

Lilian

Russ MacDonald said...

Hi Lilian,

You're welcome!

That will do it. Another color that almost always causes blow out oln the face is red; especially red hair or reddish colored skin. When I see that, I always turn the flash down by -1 ev.

Russ

Geri-Jean said...

WOW!
Thank you SO much for all you have shared here. It is helping me IMMENSLEY !! Very easy to understand!! Thanks AGAIN!!

Russ MacDonald said...

Hi Geri-Jean,

Thanks for the kind words!

Russ

play2ns said...

Russ,

Thank you so very much for your to the point explanations of the Nikon lighting system. You have been a great help to anyone who has read your comments.

Need advise on setting Nikon d200/d300 for individual and small group shots in a fairly dim environment.

I have been setting the camera to P mode and the flash to TTL with a Lightsphere 2 to eliminate shadows. The Lightsphere seems to take a lot of battery power with these settings. ISO is set to auto, that is probably a mistake!?

Please advise on how you would set the camera to get consistant results with the least amount of camera adjustments as to not miss any photo opportunities.

Thank You in advance,

Jerry
Los Angeles

Russ MacDonald said...

Hi play2ns,

I definitely would not use auto ISO. It will allow an unknown amount of ambient into your picture, which will make very unpredictable exposures.

Set your ISO on about 400 and leave it.

Then, for any shots in dim conditions set your camera in Manual mode.

Then, set your f/ stop at around f/4 for a small group.

Set your shutter at around 1/80th.

Stand about six feet away from the group and use whatever tele setting will get the whole group in. It's always best not to go too wide on your tele setting, because it makes the people on the ends of the group look wierd. Don't go any wider than about 25mm.

Let the flash control the exposure. Adjust the flash compensation up or down to get the exposure right.

With a group, you will often run out of flash power. :isten for the telltale beeps. If you run out of flash power, then your shots will end up dark. You correct that by moving closer. Then, if necessary, take off the Light Sphere and use the flash direct.

Hope that helps,

Russ

play2ns said...

Russ,

Thank You for your advise on settings for indoor flash. Do you recommend always setting the camera on M for ambient and flash on TTL for indoor group shots or would A on camera and TTL on flash give the same results?

Thank You

Russ MacDonald said...

Hi again play2ns,

I recommend using Manual mode whenever the ambient is dim, because then you can set the aperture and shutter to whatever speed you want them to be, and they cause no change in subject brightness, since that is controlled by the flash.

Camera A mode only sets the camera aperture and shutter based on the ambient light, and the minimum shutter is limited to the Flash Shutter Speed that you choose in the menus (1/60th default on my D200). So, in dim ambient, if you select camera A mode, the shutter will always go to the Flash Shutter Speed and stay there.

I often like to use 1/80th shutter to minimize ghosting. You can choose 1/80th shutter and any aperture you want if you use camera Manual mode.

Russ

play2ns said...

Russ,

Thank You very much; I will apply your advice at the next opportunity I have.

Thank You

Anonymous said...

Great Blog !

I might have missed this piece of information. When I try to shoot a reflecting surface using the SB-600, the D-200 camera flash will shoot as well, causing a reflection. How do I keep the built-in flash from going on?
I'm shooting live fish in a fish tank with the sb-600 over the tank.

Russ MacDonald said...

To Anonymous,

Thanks for the compliment!

Since the built-in flash is being used to command the remote SB-600, there is no way to prevent it from flashing, even if you turn it to --.

The flashes are the commands, and the last command sent is the 'fire' commmand, which must occur after the shutter is open for obvious reasons. Therefore, even though commands are very low power flash, this last command pulse will also contribute to the image slightly, especially if the camera is close to the subject.

This is why they invented the Nikon SG-3IR panel. It is a $12 attachment (available at B&H and Adorama and many other online stores) that clips into the hot shoe and flips down in front of the pop-up flash to block the VISIBLE light, while allowing the IR (infrared) light to pass.

Since the remote flash responds only to the IR portion of the command flashes, blocking the visible light from the commander will stop the contribution to the image from the command flashes, while allowing the remotes to function normally.

Hope that helps,

Russ

Felix said...

Thank you... I was hoping I could keep the in-built flash from shooting, the glare spoils the image. Thank you for the recommendation, will have to block the flash with a card in the mean time. You are very resourceful...

Russ MacDonald said...

Felix,

It can't be just any card (unless you allow enough light to spill around it for the remote flash. The card has to actually be an IR filter, where it allows the IR portion of the spectrum to pass through it to control the remote flashes.

Russ

Felix said...

Hi Russ,

Would you elaborate on your statement? "unless you allow enough light to spill around it for the remote flash. The card has to actually be an IR filter".
I am missing basic knowledge in the functionality of the flash vs. IR. A link to the subject will be helpful if you like. Thank you...

(I'm in the primordial learning process !)

Felix said...

Russ,

Re-read your comment... the IR and the visible light are "generated" at the same source! I had the notion that the IR light initiated via elsewhere within the camera through some small port, like my laptop. Strange that Nikon would have not thought of that, it would avoid having to open the in-built flash.

I have place my SG-3IR order...

Again, Thank you very much. Reading your Blog is like taking a well done interactive on-line course, but much better... working directly with the instructor and writer of the course. Kudos!

Russ MacDonald said...

Hi Felix,

All flashes always emit both IR and visible light.

IR light is invisible to us and also to the digital camera sensor. Visible light is what we see and it is what the camera uses for its exposure.

The Commander sends its commands out as flash pulses, so part of those pulses is visible light and part is IR.

The remotes respond only to the IR portion of the light.

Therefore, if you block the visible light from the commander with the SG-3IR panel I mentioned, the commander will not contribute to the picture. The SG-3IR panel allows IR light to pass through it and blocks the visible light.

If you use a card, like a business card, for example, it will block both the visible and the IR light, so the remotes won't work. That is UNLESS some IR light gets around the card, possibly bouncing off the walls, to trigger the remotes. This can and does happen occassionally. So you might get lucky with the card idea.

Russ

Abe said...

Again I must say that this is the best stuff I have found on Nikon lighting, by far.

So my biggest problem now seems is that while in spot metering & TTL the pot which you metering is based has really nothing to do with the flash which is based on the (undefined?) center, even when the spot is set to the center. Is this correct? seems awkward.

Russ MacDonald said...

Hi Abe,

Thanks for the compliments!

And you got it! The flash is always center weighted the same way, even if you select Spot mode on the camera, and it works very well as long as you remember to put your subject in the center (or use FV Lock).

Russ

Abe said...

Well not knowing what constitutes 'center' since I take it that the small box that is used for focus is not the same size as what makes up this 'center' - this can be a problem at times.

Also I'm not sure why the FV will always work - since the pre-flash may be on a subject that is 8 ft from the flash and the actual flash on a subject that is 16 ft from the flahs, plus they may have different luminosity, color, etc. So in these situations the flash won't match the pre-flash and result in a wrong amount of light?

Russ MacDonald said...

Hi Abe,

Have you read my blog 'TTL and TTL_BL Study'? I discuss the area that the flash metering covers. It actually covers the whole frame, but in a gradual reduced amount as it gets towards the edges of the frame.

http://nikonclspracticalguide.blogspot.com/2008/03/ttl-and-ttl-bl-study.html

As far as FV Lock goes, you just described exactly what it is for. It is for keeping the subject exposed correctly, while letting the place that the flash is aimed in the actual picture go darker.

If you don't use FV Lock, and you place your subject at the edge of the frame, the flash will meter the distant background and usually push the flash up to maximum power. This can blows out the subject, if the background is dark. What you want to happen is that the subject is exposed correctly and the background is dark.

Russ

John said...

Russ, I tried this experiment...D300/24-70 2.8/iso400/sb600 -1.0...exposure came out great. I changed out the lens to 18-55 3.5-5.6 and now -1.0 is too much. Changed this to 0 and the exposure came out great. Conclusion? The better light-gathering of the 24-70 2.8 requires less flash so I needed to compensate -1.0. However the inexpensive 18-55 was not as efficient at gather light so no need to bump down the compensation. Would you agree?

Thank you.

Steve

Russ MacDonald said...

Hi Steve,

Exactly right!

The f/2.8 lens was allowing more ambient to enter the image and it was affecting the brightness of the subject, so you had to turn down the flash a bit.

The ~f/4 of the other lens stopped down the ambient by exactly one stop, so the flash ended up perfect.

You can get around all this (if you want to) by increasing the shutter (or ISO) to eliminate the ambient from the subject. Take a test shot without flash, and if it is nearly black, then you know the ambient is not affecting the subject brightness. Then, the flash compensation should be the same for both lenses.

Russ

Anonymous said...

Hey Russ I'm very new to photography and I sport Nikon D40 with the kit 18-55 ED II lens, 55-200vr and Sb-600. I am planing on getting lightsphere II cloud/clear but the mean time from reading this blog I now understand ttl and ttl-bl. just a recommendation, I'm going to a wedding reception and planing to take group pictures of families and dancing performance. should i use P or AUTO with TTL or an ISO of 400?

Russ MacDonald said...

To Anonymous,

Let me just give you some general guidance:

1) For indoor shots in normal artificial ambient conditions, use flash in TTL (not TTL-BL), camera Manual, f/3.5 - f/5.6, 1/80th, ISO 400. Use f/3.5 (or whatever your widest aperture is) for most shots, but when you do a group shot, use the smallest aperture you can to give you the most depth of field. Always remember that the flash adds to the ambient in TTL mode, and if you allow much ambient to contribute to the subject, you have to turn down the flash compensation or you will get overexposure of the subject.

When shooting dance shots, normally keep all the settings I mentioned above, but for fun, try slow rear sync and reduce your shutter speed to around 1/20th or so. This will make the dancers all have ghost trails when they move and sometimes makes some great dance shots.

2) For shots outdoors in bright ambient light, use flash in TTL-BL, camera P mode, ISO 200. Use the flash on about -0.7 ev compensation as a starting point.

Use your histogram and flashing highlights to check you exposure.

That's about all I can think of right now. Hope that helps.

Russ

Anonymous said...

thank you RUss for you advice, my outdoor photos of my dog turned out amazing! but about the indoors ttl well i notice on my sb600 has no bounce card and diffuser like my dads sb800, should i use 45, 60, 75, 90 degrees or straight direct flash?
would you recommend Gary Fong LightsphereII cloud/clear prokit

Russ MacDonald said...

Hi again Anonymous,

I'm glad the pictures of your dog came out good!

Indoors, as long as the ceiling is white or at least a light neutral color, just tilt the head of your SB-600 at 45 or 60 degrees and pull out the wide angle lens. This will make reasonably soft light. Never point the flash directly at your subject unless you have to (like when too far away, or no ceiling to bounce from, etc.)

I highly recommend the Gary Fong Light Sphere II Cloud. It will make the light much nicer than you can get by a simple bounce. The Clear works OK, but the Cloud makes softer light.

However, the Sto-Fen diffuser is also very good and a little cheaper.

Hope that helps,

Russ

Louw said...

Hi Russ

Great content, thank you.

I do understand that the Camera metering system does not take into account that the TTL flash will add to the exposure of the subject.

What confuses me a bit is when you say: "Also, both the flash metering system and the camera metering system read the settings on the camera (ie, ISO and f/ stop) in their calculations."
and then further on: The Flash metering system just fires the preflashes and looks for the reflected light in a center weighted fashion regardless of which camera metering system you are using or what the camera f/ stop is set to." It appears to be contradictory.

What I would like to clarify is whether or not the TTL flash will change its output based upon ISO and aperture settings of the camera. Examples: If you take a shot in a pitch black room (zero ambient light) will the flash output change if you 1st set your camera to f/4 and then to f/8.

My understanding is that the flash will have to increase its output at f/8 to compensate for the smaller aperture. Is this correct?

Same scenario as above, but keep the aperture constant and change ISO from say 200 to 400. Will the flash output be reduced at ISO 400?

All questions are related to TTL Flash.

Thank you.

Russ MacDonald said...

Hi Louw,

Now that I read that again, I agree that it is not very clear.

The real genious of the TTL system is that the power of the flash is constantly adjusted so that the BRIGHTNESS of the subject is kept constant.

What I was trying to say was that the camera metering system responds to the shutter, aperture, and ISO to determine the background exposure.

The flash metering system also reads the aperture and ISO (not the shutter) for use in its range calculations that are displayed on its LCD back panel. But the flash system does not use this information for any of its flash power calculations.

Yes, the flash will change its power output as you change ISO and aperture, but the change it makes is only to maintain the subject at a constant brightness. The flash computer does not use the settings on the camera for this purpose. It uses only the change in measured power from the monitor preflashes which occurs due to changes in aperture or ISO.

For instance, if you decrease the size of the aperture by one stop, you have cut the light passing through the lens in half. Then, when the monitor preflashes are fired, and the reflected energy bouncing back FROM THE SUBJECT is measured, it will be half of what it was before the aperture was decreased. The flash computer will see this and double the flash power to return the subject to its original brightness.

To say this another way, the flash power is INDIRECTLY determined by the settings for aperture and shutter, and not by reading those settings directly from the camera.

This is the same type of thing that happens when you add a diffuser to the flash. The diffuser reduces the power of the flash by about a stop. It also reduces the power of the monitor preflashes, and the flash computer recognizes this and increases the flash power to compensate. This system keeps the subject at a constant brightness allowing you install or remove any sort of 'modifier'.

Hope that is clearer,

Russ

Louw said...

Hi Russ

Got it. Appreciate your efforts.

Louw

Anonymous said...

Hi Russ, first of all, thank you very much for a very informative blog. I've learned a whole lot about TTL and TTL-BL and its relationship with camera metering. Can you give your recommendation if you were to use an SB 400 speedlight which has some limitations compared to the other speedlights. How would you use it outdoors or indoors? I do have an SB 600 but I would like to be able to use the SB 400 to its full potential, whatever that is, because it is smaller, lighter and generally more easier to carry around. Again, many thanks for being so generous and sharing your expertise.

Kevin

Russ MacDonald said...

Hi Kevin,

On the SB-400, the main thing to understand is that it defaults to TTL-BL just like the pop-up flash.

This is exactly what you want for outdoor daytime shots and ones that are backlit, but it doesn't work as well when indoors in low ambient where you want the flash to be primary. That's when you want to switch it to regular TTL mode.

However, you will note that there is no button or menu choice on the SB-400 to put it in regular TTL mode!

The only way to switch the SB-400 into regular TTL mode (as with the pop-up flash) is to switch the camera to spot metering. Then, be sure use camera manual mode and I suggest 1/80th and f/2.8 - f/4.5.

Your indoor low ambient flash shots will become supurb!

Russ

Anonymous said...

Thanks very much Russ. I'll definitely try using manual mode for indoor shots. All the while I've been using aperture priority though I have experimented with spot metering. Many thanks!

Kevin

Focuspuller said...

Russ, this blog is amazing. No matter how many times I reread it, I learn something new or gain a new insight.

One thing is bothering me, though:

You have said the ttl system will overexpose a subject in strong ambient light situations.

You have also said the flash computer reads the preflash energy which presumably is reflected from the subject and which is under the CW area, and then powers the flash accordingly, to achieve a well-exposed subject.

In order to do this, it seems to me, the flash system must be aware of the aperture, shutter, and ISO setting on the camera, otherwise how could it know how much power is required to make a proper exposure, right?

So if the ttl flash system is measuring reflected light from the subject, which may include ambient light if it is strong enough, and it knows the aperture, shutter, and ISO on the camera, why can't it give a correct exposure?

What am I not understanding?

tlbmh56 said...

Russ
Your insight and skill in explaining flash is beyond words. I have a question that I have not seen in your blog. For an evening wedding around 8pm, do you recommend a low ambient light flash ttl/mannual shot or try to use the existing ambient light only. I will be using a 2.8 fixed aperature. Thanks Terry

tlbmh56 said...

Russ
I guess I should learn how to spell and clarify my question better. I am referring to an outdoor wedding at 8pm Thans again. Terry

Russ MacDonald said...

Hi Focuspuller,

Thanks for the nice feedback!

Very astute question! I had the same question, initially.

In fact the ambient does affect the TTL measurement - just slightly - for the exact reasons you mentioned.

However, the instantaneous brightness of the flash is several orders of magnitude brighter than the ambient, so even the brightest ambient has very little affect on the result. The effect on flash power can be safely ignored. It is best to simply assume that the flash adds to the ambient on the subject.

The aperture and shutter settings are not used by the flash (except for estimated range on the back of the flash itself). The shutter has no effect on flash power (assuming normal flash sync mode). The aperture has an effect on flash power because the TTL system adjusts the power to keep the subject at the same standard brightness regardless of the aperture setting. The aperture is a modifier just like a diffuser dome. It will modify the amount energy that is reflected from the monitor preflash. The flash metering system uses this change in energy to set the main flash pulse, keeping the subject brightness constant. Changing the shutter has no effect on the power of the flash (in regular flash sync mode).

Hope that helps,

Russ

Russ MacDonald said...

Hi again Focuspuller,

Flash shots at dusk are the trickiest of all the flash shots. In order to get the best results, you have to constantly change your approach based on the background.

The dynamic range of the still bright sky and the dark ground is beyond the capability of the camera. If you have sky in your picture, you have to fire the flash at a fairly high power to light the subject enough to bring it within the dynamic range of the sky, so the camera can capture both. This is where TTL-BL really works well. It will balance the brightness of the subject to the sky. That's how to capture a sunset/sunrise and a subject in the foreground and get both exposed properly. Sometimes I switch the camera to manual mode and spot metering to meter the sky just behind the subject. Then, I switch back to matrix metering so the TTL-BL can be selected, and let that set the power of the flash to balance the subject to the sky.

However, if you don't care about preserving the sky, you can simply switch to regular TTL and set the camera to about 1 stop below the ambient (without the sky). This will give perfectly lit subjects and a slightly dark ambient around them, but the sky will be blown out.

You have to keep thinking on every single shot when shooting at dusk or twilight.

Focuspuller said...

Thanks, Russ, for the reply.

I get that strong ambient and ttl are a bad mix. I just can't quite grasp how the flash system comes up with a proper flash duration which somehow results in a proper exposure, if camera parameters are not taken into account.

And I believe your second reply to me should have been addressed to tlbmh56, although it was very informative. I didn't know ttl-bl works in manual mode.

Anonymous said...

Thanks Russ, for your great blog! It already helped me a lot! Pictures that I make indoors have never looked better, thanks to your advice. But I have big problems to get consistent results when I shoot in bright contrasty daylight. I get a lot of underexposed images. Sometimes both background and subject are totally underexposed, when I use TTL-BL. Mostly I shoot in Aperture priority or Shutter priority mode. And I use Auto-ISO. Is that the reason I get very inconsistent results? Most of my pictures I made outdoors are with ISO 200.
And sometimes in bright daylight my flash is too weak to lighten the subject. I see -3.0 or an other number on my flash screen. I use a SB-600. What do I need to do then!?? But then again, it's very inconsistent. Sometimes I take a picture and its good. And then I take another shot of the same, and it's underexposed. I don't get it. What am I doing wrong!?
The biggest problem is the underexposure.
I hope you can help me. Aren't you getting tired of all us asking you questions? Haha!

Greetings from Holland,
Jesse

Russ MacDonald said...

"I just can't quite grasp how the flash system comes up with a proper flash duration which somehow results in a proper exposure, if camera parameters are not taken into account."

It does it because it is a closed loop system. It works the same as any modifier, like a diffuser. Anything that affects the monitor preflash strength will also affect the power of the flash. If you install a diffuser, the monitor preflashes will be weakened, so the system will automatically increase the power of the flash to compensate. Same thing happens if you decrease the aperture. That will make the monitor preflash strength less, so the flash power will be increased accordingly.

The system never knows what the reason is for the decrease in monitor preflash stength, but it doesn't care either. It just increases the flash power to compensate.

Russ

Russ MacDonald said...

Jesse,

Thanks for the nice feedback!

Shooting in bright contrasty daylight is difficult. The problem is compounded when in the shade, with bright daylight outside the shaded area included in the background. Then, bright spots in the background greatly reduce the aperture, leaving the subject dark. The flash is not strong enough to bring the subject back to correct brightness.

There is no easy answer. Sometimes, I switch to camera Manual mode, and set the aperture and shutter on one setting that makes underexposes the subject by about 2 stops. Then, the flash will bring the subject back to normal brightness. And the bright backgrounds won't keep changing the camera settings.

I always use TTL-BL and Matrix metering, however.

I never use auto-ISO. In bright daylight I simply set the ISO to 200 leave it there.

I also remove all diffusers and point the flash directly at the subject.

Another thing to avoid is FP High Speed Sync when in this type of situation. It reduces the power of the flash a lot, and it is often better to use S mode with the shutter at 1/250th or less (which will keep the camera in normal sync mode) and let the camera adjust the aperture to maintain the correct exposure.

Hope that helps,

Russ

Anonymous said...

Hi Russ,

Thanks for your fast response. I always use a diffuser, so I think I'm going to remove it when I'm outside. And I always use FP (thought that was a handy feature, but now I have my doubts, haha). And I use auto-iso, as you know. So I think my problem is solved. I'm going to try out really soon. Today I had the same with dark subjects, and they were all taken with a shutter speed faster then 1/250.
And I saw today my pictures looked very dark in bright daylight. And then I reviewed them in the shadow, and they were all correct exposures, so that's another reason. Maybe I'll just need to check the histogram! Haha.
Thanks again for your very professional comments and good luck with your blog!

Greetings,
Jesse

Focuspuller said...

Hi again, Russ,

I thanks for the extended answers on this blog.

I think I've got it, finally. The flash durations are so much shorter than any possible camera shutter speed, the issue becomes "what happens during the remainder of the shutter-open time?" If there is little or no ambient, the exposure will be correct. If there is significant ambient, it will add to the exposure and cause overexposure of the subject.

Am I correct?

Anonymous said...

Russ:
I didn't see a topic about this so I just posted the question here. Hope that is ok.

Question about the zoom function on the SB flashes ( mine is SB-600).

When I put on my 70-300 VR and zoom, the flash shows me the zoom distance just fine. But when I put on the 50mm f2.8, the zoom number on the flash reads 14mm. Is that right? Why didn't the flash pick that up from the lens? Should I set it manually each time I use this lens? (to 50mm or 75mm)??

Thanks again,
Richard

Anonymous said...

Russ:

Never mind. I took the flash off the camera to change batteries, and when I put it back on, the Zoom Head correctly read 50mm. Don't know what was up with that, but it is fine now.

Thanks,
Richard

Russ MacDonald said...

Hi Richard,

I use my 50mm f/1.8 with my SB-600 all the time, and it always goes to 50mm.

I think you may have pulled out the wide angle lens. That is what forces the flash to 14mm.

Russ

John Meyer said...

Great blog!

Russ MacDonald said...

Thanks John!

Russ

Didzis said...

Russ!

In this blog entry you write:

"The size of the main flash center metering area is about half the size of the area between the four nearest focusing sensors around the center sensor."

I wonder if this really is true. From your other article # 9 "TTL and TTL-BL study" it appears that the flash metering area is no THAT small - and that it is more like true CW pattern.
Or am I misreading this - because the way I understand the above text is that on a D700 the area of "about half the size of the area between the four nearest focusing sensors around the center sensor" is more like a very 'fat' spot meter - very small indeed.

Russ MacDonald said...

Hi Didzis,

My characterization of the size was based on the placements of the focus areas in the Nikon D200 camera, which are much farther out from the center than with the D700.

However, since I wrote that, I have found that the flash metering actually extends all the way to the edge of the frame in a center weighted manner. And the sensitivity of the metering decreases drastically by the time you get to the very edge of the frame, which causes the power of the flash to increase greatly to try to light the background.

The end result is that if the subject will be off-center, and the background is dark, you must use FV Lock to get accurate metering on the subject. I have written another blog on FV Lock.

So, thanks to your observation, I will modify this blog to clarify this point.

Russ

Itamar said...

Ross,

first of all, congratulations for the great job in this blog.
The most useful source of information regarding to Flash use and photography in general.
You should really write a book.

The TTL system usage of the center part of the frame is very cumbersome.

99.9% of the times the subject is not in the dead center.

I think it should be very easy to Nikon to use the AF area as the flash metering point for example, using the same controls that already exists.

Regards,
Itamar

Russ MacDonald said...

Hi Itamar,

Thanks for the nice feedback!

The TTL metering is Center-Weighted. This means that it uses the whole frame, but the emphasis is on the center. The subject does not have to be dead center. It really isn't cumbersome, and it has to work that way if the subject is to be lighted by the flash separate from the background. In other words, the flash needs to know that things toward the edge of the frame are to be treated as background.

Russ

Anonymous said...

Hi Russ,

Thanks for the execellent information on your blog !
I have a question about TTL flash inside. You mention ISO 400 1/80 f3.5 for dim situations. How do you meter if you are going to shoot a wedding indoors but with daylight trough the windows? Are you underexposing the ambient (how much) and make your flash the dominant lightsource or do you fill in the scene? I normally meter the ambient en underexpose 1 to 2 stops and than use the flah in TTL mode. In very dim inside situations and bright outside it's clear how to set the camera and flah but what to do in de situation indoor in daytime?

Another question is about ISO. The new D700 / D3 camera's have very high ISO performance. Is it usefull also in dim situations to use a higer ISO such 800? The reason i can think of is the flahs don't have to work so hard and it's softer for the people you are photographing?

Thanks for the help !

Regards,

Gerald de Blok

Russ MacDonald said...

Hi Gerald,

The situation where you have strong outside light coming indoors is very difficult. You can always try measuring the ambient and setting the camera at -2 stops with TTL flash, but with this situation, the ambient will probably be varying all over the map.

So, I normally switch the camera to A mode, select Auto FP Sync, and use TTL-BL. Then, the flash will set its power based on balancing the subject to the ambient. This usually works pretty well for quick grab-shots.

However, for fixed group poses with bright ambient, it's usually best to measure the ambient for that location, set the camera at -2 ev, and the flash on TTL and -0.7 ev.

As far as boosting the ISO when shooting flash, that will reduce the flash power and extend battery life. However, you have to be very careful of close-up shots where the flash can't reduce to a LOW enough power. I think ISO 800 is about as high as I would ever try. I think ISO 1600 will definitely blow out some close-ups.

Hope that helps,

Russ

Mark said...

Russ,

Just wanted to add my voice in thanking you for this wonderful blog. Last night took some night shots of a soccer team and used TTL, Manual, 1/60, F4 and the shots were perfectly exposed for the players. The background was dark, the players well exposed. For years, have tried to understand how to use my SB-800 ... with intermittent success. Now I feel I have the knowledge to be successful. Thanks again.

Mark

Russ MacDonald said...

Hi Mark,

Yep, that's the way to do it. However, I'll bet you got some serious ghosting at 1/60th shutter. You get ghosting from the ambient light when the shutter speed is too slow.

Another thing you may have noticed is that the ghost trails go in the wrong direction. You have to use Rear Sync to make the ghost trails appear on the correct side of the action.

I think I would have used 1/125th shutter or so to reduce the ghosting. 1/60th is really too slow for action unless your goal is to make ghost trails. Sometimes you do want that, but you only want it on a few shots. A little goes a long way.

Most of the time, you want to freeze the action (which the flash will do). To do that you have to eliminate the ambient with a higher shutter.

Thanks for the great feedback on my blog!

Russ

Mark said...

Russ:

I should have clarified. The shots of the soccer team were static ... pictures taken after the game with the team posing. So I don't think I had any issues with ghosting. What I did find with these shots, after taking them into Lightroom, was that I probably should've used F5.6 or F8 to increase my DOF ... as the players were aligned into 2 rows and I think I could've gotten it a bit sharper. I also found that there was some background shadows that showed up ... so also maybe increased the shutter speed a bit to further reduce the ambient light. Does that make sense? Thanks.

Russ MacDonald said...

Hi again Mark,

Yes, I misunderstood.

Team portraits should be fine at 1/60th, as long as the ambient light somewhat matches the flash. For instance it will work great outdoors.

However, indoors under incandescent light, you might want to use a higher shutter speed to eliminate the ambient. Otherwise, you will see a color shift from white to yellow from the front of the group to the back.

Yes, f/8 would increase your depth of field and make the focus sharper across the group, but that is getting close to the limit of the smallest aperture you can use with the SB-800 when using a diffuser. In fact, you may have to remove the diffuser to get to f/8.

Russ

Mark said...

Russ:

The pictures were outdoors, at night, using "M" camera mode and TTL flash mode. Not sure I understand your comment about the diffuser. I use Demb Flip-It, and it worked well during these night shots. Please expound. Thanks much. And Happy Thanksgiving.

Mark

Russ MacDonald said...

Hi Mark,

I meant only that the diffuser reduces the effective power of the flash, so you can't use as small an aperture when using it. Then, if you want to use f/8, you might have to remove the diffuser to increase the effective power of the flash enough to make a proper exposure.

The Demb Flip-it will cause the flash to reduce in power by about a stop and a half. For a large group, this will mean that the smallest aperture you have enough flash power for is about f/5.6.

Without a diffuser, the flash has enough power to use about f/11.

Happy Thanksgiving!

Russ

Mark said...

Russ:

Thanks for this explanation. Know I've read this in the blog, but what is the relationship of flash power to the camera settings? I thought if using "M" camera mode and TTL for flash, the flash power is determined independent of the camera settings, which control the ambient light exposure. But from reading your previous entry, think there's more to it. If you could please 'illuminate' this again, would be greatly appreciated. Thanks.

Mark

Russ MacDonald said...

Hi Mark,

My previous entry was about diffusers. Diffusers attenuate the flash which decreases MAXIMUM flash power, which reduces the range of the flash. As long as you are inside the range of the flash, the SUBJECT brightness from the flash will remain constant.

When you are in camera Manual mode, the only setting on the camera that will change the subject brightness is the EV button. That will increase or decrease flash power to brighten or darken the subject without affecting the other camera settings (since you are in camera Manual mode). This button works just like the FEC button on the flash in this case.

Russ

Alfonso said...

Hi Russ!

First of all, thanks a lot for this blog. I have been taking pictures with natural light for quite some time but I just got my first CLS-compatible flash (an SB900) two weeks ago. Nikon's documention is very confusing and not the least bit scientific.

Your blog has helped me a lot. Even with that, I have noticed a thing which could be more accurate (maybe you just decided to avoid overcomplicating the exaplanations).

"The Camera metering system does not take into account that the TTL flash will add to the exposure of the subject. You can prove this by taking a picture, with and without flash, with the shutter set at a fixed amount. Set the camera on S mode and the shutter to 1/80th and let the camera pick the f/stop. The f/ stop that the camera selects is the same whether the flash is on or off."

Only partially true. While your statement is true for shutter-speed priority, that is not always the case for aperture-priority. Depending on the camera settings, there can be an upper (sync speed if FP is not enabled) or lower (slow sync) limit.

I hope you don't find my comment offensive. I just realised about these things while putting your blog entries in practice and though others might find them useful too (i.e. I'm not trying ot play smart-ass here :)).

Fons (from Spain)

Russ MacDonald said...

Hi Alfonso,

You are right that there is a maximum shutter speed (Flash Sync speed) that might come into play if you use Aperture Priority in bright ambient light (ie, daylight), and that could make the shutter speed different depending on whether the flash is on or not.

However, if that happens, then you are always going to end up with ambient overexposure. In other words, if you ever allow this situation to occur, you have made a mistake. You have to go back and stop down the aperture to be able to obtain a correct exposure. Once you select an aperture that allows a correct ambient exposure, then the shutter will no longer try to exceed the sync speed, when the flash is turned on, and the camera settings will remain the same whether the flash is on or not.

This limit is a physical limit caused by the design of the shutter. The curtains in the focal plane shutter in my D200, for instance, take exactly 1/250th sec to open or close. This is why the shutter is limited to 1/250th sec when using the flash. It has nothing to do with the exposure metering.

In fact, this is exactly why in another one of my blogs on TTL-BL, I recommend using P or S mode whenever shooting in bright daylight. Then, you never run into the shutter flash sync limit, and you never get overexposure.

You are also right that there is a minimum limit on the shutter when shooting flash in dim ambient light (unless you are using Slow Sync). However, again, this has nothing to do with the exposure system. This limit is imposed to avoid ghosting blur from camera motion when handholding the camera. 1/60th is the default, because that is the accepted slowest speed that the average person can hold the camera still enough to avoid ghosting when shooting flash.

Anyway, the point I am making in this blog is that the camera exposure system doesn't reduce the ambient exposure when the flash is turned on. This means that the flash will add to the ambient exposure so in bright ambient light where the flash shutter speed is not being imposed, you have to turn the flash down to avoid overexposure in bright ambient conditions.

In dim ambient conditions, the Flash Shutter Speed limit will be imposed, so you don't have to turn the flash down, since the camera will underexpose the ambient anyway - but the shutter speed limit has nothing to do with the camera trying to get a correct flash exposure. It's to limit ghosting.

Sorry for the long-winded answer, but your comment was a good one!

Russ

Anonymous said...

Hi Russ,

I do not have a flash yet, intend to get a sb600 soon. Yet your suggestions make a great difference with my SLR's pop-up flash photographs (with spot metering).

Thanks,
Dinesh

Russ MacDonald said...

Hi Dinesh,

I am glad you find my work helpful!

You will love the SB600 when you get it.

Russ

Clearlight said...

Hello Rush. I am learning a lot reading your blog. I would like your advise based on your experience as a wedding photographer about the following. I am going to shoot some photographs in my friends wedding, and though I dont intend to emulate proffesional job I would like to make decent photos and learn and practice at the same time.
I have got a D80 with an SB400 and I dont expect much light inside the church, actually not natural, under wood ceiling and white walls. Tiny church. I have only these lenses: Nikon 16-85 3.5-5.6 VR, Nikon 24mm 2.8D and Nikon 50mm 1.8 not D.
How should I "dress" my D80 for the wedding and should I set the SB400 for TTL direct flash?
Just pretend some shoots inside. Which are the most important moments in a wedding? What lens for outside group and personal portraits in TTL-BL? Any proffesional recommendation I will strongly apreciate.
Thanks a lot for your help in this question and for your blog full of very very clear information.

Russ MacDonald said...

Hi Clearlight,

I recommend you read the rest of my blogs. I cover all the questions you are asking. :)

However, switching your SB400 between TTL-BL to regular TTL is a little tricky (and I have not discussed this in any of my blogs). The SB400 is the only external flash that doesn't have menu option to switch it between TTL and TTL-BL modes. It switches based on the metering mode of the camera.

In fact, The SB400 it works exactly like the pop-up flash. If you want it to be in TTL-BL mode, select Matrix metering on the camera. If you want it in regular TTL mode, select Spot Metering on the camera. That's the only way to switch it between TTL and TTL-BL and it is not explained in any Nikon documentation.

Now, for shooting the wedding let me quickly summarize:

1) Inside the church, in dim light, use your camera in Manual mode, ISO 400, aperture wide open, shutter 1/80th. Then select Spot metering to force the flash into regular TTL mode.

Don't be intimidated by flash photography using the camera in Manual mode. The TTL flash will control the whole exposure as long as the ambient is dim enough that it doesn't contribute to the exposure.

2) Focus will then become the most difficult part of the job. I recommend single area focus with single servo. Place the focus area on the subject's face, half-press the shutter to focus, recompose to place the subject properly in the frame and complete the shutter button push to take the shot. If the subject is moving toward you, do this very quickly to make sure that the subject doesn't walk out of the depth of field and become soft.

Use this same focusing technique for all your shots, inside or out.

3) Then, for outside pictures in bright daylight, switch the camera to P mode, ISO 200, Matrix metering. The main exposure in bright will be from the ambient light, and the flash will be adding fill in TTL-BL mode.

There are lots more details about this in my other blogs.

Hope this helps,

Russ

Russ MacDonald said...

To Clearlight again,

I forgot one thing.

Use your 16-85 lens for everything, but zoom it all the way in when inside the church, because it only opens to f/5.6 at that setting.

The primes are much faster (f/2.8 and f/1.8) and they will allow better exposures in dim light, but they will also make it difficult to achieve and hold focus especially when the subjects are moving.

If you want to try the primes, use Continuous Servo (AF-C) mode and Dynamic Area mode. That should give you better focusing than the method I mentioned in my previous answer to you..

Russ

Clearlight said...

Thank you very much for your cute answer and your kind help, Russ (not Rush sorry). I will read all the blogs. Cheers from Madrid.

Robbie said...

Hey there, this stuff is gold.

I've been running into a few traps now that I'm getting out and shooting every weekend. I have decided to nut out every aspect of the flash, so I can start getting consistent shots.

I really only ever shoot in Manual mode. I'm scared of the auto modes, except maybe Ap mode, thats handy sometimes.

I recently did a wedding and it went horrible for the indoor shots in the hotel room. I had severe underexposure on one photo and severe over exposure on another.

Typically when I shoot flash, I set the Aperture I want first. Then I set the shutter speed on Centre weight, on the background ambient light. Adjust the ISO to get to min 1/30 shutter if dark. Then I set to spot meter, and I think I'm using Ittl mode. And then I trial and error using exposure control. IM not sure if this is the best way of doing it.

You wrote that the idea of fill flash is to reduce shadows, and to keep natural light. A shoot I just did on the weekend, it was your typical 3pm shoot in the shade, plenty of greens, and yellows. I used my technique, but then built the flash up on the subjects, so that yes it did make them stand out, the eyes came out nicely lit, and it was a vibrant shot. I find sometimes when I use available light my photos look a bit flat. My main reason for doing this is I once had a group photo of 40 people in the day. I used the shade against the sun. And I found it hard to bring out detail in any of the subjects. Where had I have used flash, I could have brought out their faces better. Is this a problem, to be doing for developing a style. particularly if I'm getting into family portraits. Basically its my intention to add a soft light source to the subject to take it out of shadow.

I do need to start understanding how to effectively start using the flash unit however. Your posts are awesome. And I'm going to keep going through them to understand in greater detail.

Thanks

Russ MacDonald said...

Hi Robbie,

Thanks for the nice feedback.

I think you are asking me how I shoot flash. Have you read my Cookbook blog? Here: http://nikonclspracticalguide.blogspot.com/2009/12/camera-flash-cookbook-for-any-lighting.html

One thing I never mess with once I set it is the ISO. I leave the ISO fixed at 200 for outsoors shooting fill and 400 for indoors shooting flash primary.

Russ

Robbie said...

Hey Russ

Thanks for the comments.

I'm so glad I stumbled on this site, Because for Three months, I had no idea what I was doing, I'm not even sure I was always using ttl mode.

Thanks for the link, it explains a few things. The only time I really do change the ISO I should explain is when I'm indoors, and there is nice brickwork, like the wedding I just did, So I used an 105mm f2.8 at 1600ISO - 2400 A bit grainy but nothing to really wory about. And I just used flash when needed. I had to up the ISO to preserve the ambient light.

I followed your guide, and thats pretty much how I've learned to do it, metering in manual mode, sometimes spot meter, its a great tool when determining the difference in brightness of various points in the sky for example.

For outdoor shots, usually I would metre the background, If in manual mode, I can set the aperture, and throughout the shoot, switch the shutter speed between 125 and 500s to adjust ambient exposure. Then add flash. Now that I understand how ttl works, the shots might be more consistent. Quite often shooting in shade and adding a little bit of fill flash, I find myself having to dodge and burn the eyes.

When I get my photos up on my website, I will post a link so you can see what I mean.

nikonrick said...

Russ:
I found this fabulous blog last year and soaked it up...as best I could.

Now, I'm back to try and get what I missed before.

I LOVE this blog.....thanks so much for all your time and effort in writing it and keeping it up to date.

One thing you said 'below' that shook me to the core....I never realized that taking the flash off camera kills the TTL-BL mode. AMAZING what you learn when you read things twice, huh????

So, question is.....now what? If you love outdoor portraits, and we've all been told to get the flash off the camera, so I bought a light stand, small soft box, umbrella, etc.....now what do I do with all that to "get back" what I lost by taking the flash off camera????

Confused.......Richard

nikonrick said...

Russ:

By the way, I shoot with a D700 and SB-900 if that makes any difference.

Thanks,
Richard

Russ MacDonald said...

Hi Richard,

Thanks for the nice compliments!

Well, when you take the flash off-camera, you are back to the way we always did fill flash before TTL-BL was invented! It's not hard, and many experienced photographers still prefer to handle it the 'old' way.

Here are the steps:

1. Camera Manual mode, Spot metering, ISO=200.

2. Point the camera at the background and zero the built-in meter with the shutter and aperture settings.

Note: You might also want to check the brightness of the subject, however, to make sure it is darker than the background. When shooting fill you are always brightening the subject to equal the background.

3. Take a test shot without the flash. The background should be exposed correctly. Use the histogram and flashing highlights screens. The subject should be darker than the background.

4. Now comes the trial and error part. Turn on the remote flash in TTL mode and add negative compensation in the Commander. Take a wild guess at -1.0 FEC to start. Remember that TTL flash always adds to the ambient on the subject, so it has to be turned down to avoid overexposing the subject. The amount is determined by trial and error.

4. Take a test shot. Check the histogram and flashing highlights screens. The histogram should be distributed fairly evenly across the graph without going up the right edge. There shouldn't be any flashing highlights on the subject or the subject's clothes. Flashing highlights in the background may be OK depending on where they are.

5. Adjust the flash up or down using the Commander to set the brightness of the subject to where you like it.

That's it!

A lot easier with TTL-BL, isn't it? But then the flash has to stay on-camera.

Regards,

Russ

nikonrick said...

Russ:
Thanks for the update! That's pretty much what I thought, but it's nice to hear it from someone who knows.

Thanks again for the great info.

Richard

Carsten W. said...

Hi Russ,

thanks a lot for all your work here in the blog. I read all your blogs in the last 4 weeks during my holiday in brazil.

I learned a lot from you and i also did a lot of exercises the last week.

I still have a question. In the past 7 months i always have a problem with the Flash Fast Series. I use a D300 with the SB900 in TTL Mode . Doesn't matter if i use FV lock or not. I did not use FP High Speed Sync and normal values like f3.5 and 1/500 shutter outdoor in bright sunlight or normal sunlight.

When i use "slow" 3fps and make for example 5 pictures then all the pics have different light of Flash on the face of the person. Also when i use 6fps or more. The Flash Button on the back of the SB900 did NOT blink (so still enough power in it). I did not use an compensation of flash or camera. All are zero.

Most often the first picture is the brightest and the next is a little more darker. But significant !

Thanks for your help
Carsten (Germany)

Russ MacDonald said...

Hi Carsten,

Yes, this effect occurs in all speedlights. Let me try to explain it.

The reason this happens is that the discharge curve of a capacitor is not linear. In fact, the voltage across the capacitor is proportional to e**(-t/RC) where RC is 1 time constant. The flash is designed to work in the first 3 time constants. The last two time constants the voltage drops to a level too low to keep the flash tube fired. That is roughly the half-power point. This is all related closely to the blog I wrote about the flash actually firing longer than the shutter open time at the flash sync speed.

Anyway, the math to show this requires differential calculus, which I won't try here. Just suffice it to say that as the capacitor discharges, its voltage decreases nonlinearly, and this means that as you progress in time farther and farther down the discharge curve, you transfer less and less charge per unit time.

The calculated power setting is in units of time. In other words, when you push FV Lock a preflash is fired, and the power required is calculated in units of time and that number is stored.

For example, lets say the flash metering system determines that the correct exposure would result by firing the flash for 1.0 ms.

If the flash fires for 1.0 ms when the capacitor is fully charged, the exposure will be correct. However, if the flash is fired again before the capacitor can recharge, then it will be farther down on the discharge curve for 1 ms, and slightly less charge will pass through the flash tube because (C*dv/dt) has changed. This will reduce the number of photons produced which decreases the exposure.

So, it is normal that as you burst-fire the flash consecutively, without giving it time to recharge, each flash will be slightly less powerful than the one before, even though the flash will fire for the same period of time each flash.

However, if the required flash ON time for each pulse in a burst is only 10 us (microsec), then the difference in power between successive pulses would be so small it would be negligible. They could be treated as equal for all intents and purposes.

It's only when you use a large percentage of the stored charge that successive pulses show noticeable decrease in power.

Hope that is clear,

Russ

Carsten W. said...

Hi Russ,

WOW ! Fast answer, thanx a lot !

So in other words:
I did not make a mistake when i fire a serie of 3 or 5 pics. I really thought the SB900 can do a serie of 3 or 5 pics with equal light fire.

So, that means when i want to fire a serie of 5 pics in an outdoor party or beach i should turn OFF the FLASH to have always the same exposure ?

But when you see in the TV the Photographers fire more than one pic with the flash ON. Or do they use different technics.

Regards,
Carsten

Russ MacDonald said...

Hi again Carsten,

I shoot weddings, shoot multiple flash burst all the time. I just increase the exposure in LightRoom during post processing.

However, there are some photographers who use a Quantum Turbo battery pack. This pack applies high voltage directly to the capacitor on the SB-800 through special high voltage connectors on the front (I think it is the same for the SB-900).

This will recycle the flash in a half a second or less (most of the time), so you get much less exposure difference from image to image taken with flash bursts.

One big problem I have heard about with the SB-900 is that it will quickly overheat if you fire in bursts.

Russ

seablister said...

If you were to describe the way a flash works in A mode, would it be quite similar... allowing for the pre-flash back to camera sensor vs actual realtime flash back to the flash sensor differences?

I only ask because I have a repeating situation where I shoot a moving subject with very dark background as you have in tis example and I really don't have time to use the FV function. Thought the A mode might offer an opportunity.

My question really focuses on the pattern of sensitivity A vs TTL. TTL is very center weighted. True of A, as well?

Thanks

Russ MacDonald said...

Hi Seablister,

The Flash A mode doesn't use the preflashes, but the AA mode does. Both use the small thyrister eye on the front of the flash to measure the reflected energy and shut the flash off at the proper time.

The metering pattern with flash A and AA is still center weighted, but much less so than TTL. So, yes, I think flash A or AA mode would have given good results for my example in this post.

However, it is easier, I think, to use FV lock than switch the flash to AA mode.

I wrote a separate blog on flash A and AA modes here:
http://nikonclspracticalguide.blogspot.com/2008/07/13-sb800-and-aa-modes.html

Russ

Carsten W. said...

Hi Russ,

thanks again for the nice and fast answer.

1. By the way i use Aperture 2. I like both tools but Aperture 2 is more MAC style for me :-) You said you increase the exposure in LR. From all what i read and understand here of background brightness and TTL Flash Computer. If you exposure in post processing to bright the subject (because the Flash Power was not there/enough again) you will also brighten the background at the same time, correct ?

2. SB900
I like my SB900 very much and i did never recognize this overheating problem (disused in some forums). Even if i shout in bright daylight with FP or in very dark environment the series of 10 or 15 pics, it was only that the RED lamp on the back of the SB900 was flashing.
I use the Sanyo eneloops with the flash.

Regards,
Carsten

Russ MacDonald said...

Hi Carsten,

Yes, if you increase the exposure in post processing (on the raw image) it acts similar to increasing the exposure in the camera (except noise increases). Yes, the background gets brighter.

I have heard on the SB-900 that using Eneloops reduces the overheating problems.

Russ

Ulf said...

Hi!
Sorry for my English.
Thanks a lot for these wonderful articels. They help me so much.

But there is something not clear to me:

You wrote the amount of the Flashlight does not affect the ambient light??

I thought this is the big advantage of i-TTL.
That I can make what I want (shutter Aperture Iso) and the i-TTL will mange the Light for my subject (when it is possible)
So if the ambient light is to bright (or I set a long shutter) the flash would reduce.

What didn't understand?
Thank so much.

Ulf said...

Hi!
Sorry for my English.
Thanks a lot for these wonderful articels. They help me so much.

But there is something not clear to me:

You wrote the amount of the Flashlight does not affect the ambient light??

I thought this is the big advantage of i-TTL.
That I can make what I want (shutter Aperture Iso) and the i-TTL will mange the Light for my subject (when it is possible)
So if the ambient light is to bright (or I set a long shutter) the flash would reduce.

What didn't understand?
Thank so much.

Russ MacDonald said...

Hi Ulf,

Actually, what I wrote was that the amount of ambient does not affect the power of the flash.

I think you do not understand why iTTL always meters the same regardless of the ambient.

The fundamental reason for this is that the flash is several orders of magnitude brighter than the ambient for the very short time it is on, maybe 1000 times brighter, and when it is metered by the separate flash metering system, the ambient is so small that it has virtually no effect on the flash power calculation.

Therefore, the same flash power will be calculated in a pitch dark room as in bright daylight.

The iTTL metering always assumes that there is zero ambient, like shooting flash in a dark room. It sets the power of the iTTL flash to provide all the illumination of the subject. This is what is meant that the flash is primary on the subject.

However, if there is significant ambient light, then it will add to the flash contribution and you will get overexposure.

This is why iTTL-BL was invented. In iTTL-BL mode, the ambient is measured by the camera metering system (must be in matrix or center weighted mode) and the flash computer uses that information to balance the flash contribution to the ambient. This is why you should always switch to iTTL-BL mode whenever the ambient is bright.

Hope that helps,

Russ

Ulf said...

Hi! Russ!
Wow thanks for the fast answer.

This means in normal i-TTL Mode the flash power ALWAYS is calculated for the subject when the subject is in total dark conditions?

So even if I wouls increase the shutter from 1/60 to 1/15 for example the flash does the same?
So I have to reduce manually the flashpower -2?

And about BL.
I thought BL is just to use if the backlight is BRIGHTER than the subject. Not even if the backlight is just a little bit brighter but darker or the same level than the subject.

thanks a lot. it's great to stay in contact with a real professional.

Russ MacDonald said...

Hi again Ulf,

"This means in normal i-TTL Mode the flash power ALWAYS is calculated for the subject when the subject is in total dark conditions?"

Yes.

"So even if I wouls increase the shutter from 1/60 to 1/15 for example the flash does the same?"

No. The shutter speed does not affect the flash, but it does affect the ambient contribution to the exposure. It all depends on how bright the ambient is.

Please read all of my blogs and the questions and answers after each one. I explain this in detail.

"I thought BL is just to use if the backlight is BRIGHTER than the subject. Not even if the backlight is just a little bit brighter but darker or the same level than the subject."

That is true, but almost all of the time, when the ambient is bright, there are shadows on the face of the subject that are darker than the background. The flash then brightens those shadows to equal the background.

It is almost always best to use TTL-BL when the ambient is bright.

If you use regular TTL in bright ambient conditions, you will get overexposure which you will have to control manually with the flash compensation.

Russ

Ulf said...

Hi!
Thanks a lot for your comments. For sure I read all of your flash articels and I think they helped me very much.

So i have to realize when the shutter is very long that my subjekct could get the ambilight too an have to reduce the flashpower. (I thought CLS is abe to manage this alone. becouse the camera knows every data and value)

Is it completely the same for my gear? (Nikon D300s with SB-900 (and SB-600 as second slave)


What I'm so ipressed is the Style of your fotos. The LOOK.

The look that professionell I'm not able to do.
I can't understand. You sometimes really just use one Flash (and sometimes the real light) some of my pictures are ok, but not that good. Your pictures are sharp and powerful. Very open aperture values I think but my fotos have them too.
Did you do much on RAW Converter (I am using Aperture3) or in Photoshop?

How can I get this look. lots of professioalas have this look. It looks like the picture in the magazines. An my pictures are nice but not that feel.
thanks a lot.

Russ MacDonald said...

Hi again Ulf,

You wrote:
"So i have to realize when the shutter is very long that my subjekct could get the ambilight too an have to reduce the flashpower.

This is correct when using regular TTL.

(I thought CLS is abe to manage this alone. becouse the camera knows every data and value)"

TTL-BL will do what you want automatically. If you allow a lot of ambient to contribute to the exposure, TTL-BL will automatically reduce the flash power for you and prevent overexposure.

It's only if you try to use regular TTL and a very slow shutter that allows the ambient to contribute significantly, that you have to reduce the flash power manually.

However, it would be very unusual for you to use a really slow shutter to allow the ambient to contribute significantly when in low ambient conditions. Normally in low ambient conditions, you want the flash to overpower the ambient and provides all of the exposure. That's what regular TTL is for.

Russ

Ulf said...

thank you thank you thank you.

Is it may possible to get tips about your look?


AND Is it the same with my gear? (SB-9000 D300s) or did change anything in the last few years=

Russ MacDonald said...

Hi Ulf,

I think you are asking me what gear I use.

Well, my main camera/lens is a D3 with the 24-70mm f/2.8 lens.

I use four SB-800's and two SB-600's in umbrellas for my group and formal shots.

I also use my D200 at weddings with my 70-200mm f/2.8 on a tripod to shoot available light from the back of the church.

My assistant uses my D200 with a 17-55mm f/2.8 to help me shoot the receptions.

I also have a D70 with an 18-70mm f/3.5-4.5 as a backup.

Russ

Ulf said...

Hi thanks.
And with all the equipment (even the newer D3 (and so my D300s to) it works the same way?

No I really mean the LOOK on your Photos.
This impressed me.
Is there a lot of Photoshop or is it the Light or what could it be? It looks like Studio pictures but the most is made outdoors with a small setup D3 or D200 aand one or 2 Flashes.
I'm ver impressed.

Russ MacDonald said...

Hi Ulf,

Thank you for the nice compliments. I used photoshop on a few on my pictures to give them special effects, but most are just they way the looked out of the camera.

The 'studio quality' look comes from knowing how to diffuse the flash properly. A lot of credit goes to the Gary Fong Light Sphere that I use. Last weekend there wasn't even room at the wedding to set up umbrellas, so I shot all the formals with only the LS, and they came out pretty good.

Russ

Russ MacDonald said...

Ulf,

First, have you read my blog here: http://nikonclspracticalguide.blogspot.com/2009/03/15-when-is-full-power-flash-not-full.html ?

When you set the flash sync speed to 1/320th and set the shutter speed to 1/320th it simply cuts off more of the tail

With more of the tail cut off, the power is reduced signitifcantly more than at 1/250th.

On the D300 with the shutter speed at 1/320, the maximum flash power is nearly the same in normal sync as FP sync. So there is really not much advantage of using the 1/320th flash sync speed.

My D3 does not even have the 1/320th flash sync.

Russ

Ulf said...

Thnaks.
So it is not real 1/320?
It is a mix between FP and normal?
thanks

Russ MacDonald said...

Hi Ulf,

Your question is way off topic for this blog 'Nikon TTL Flash Metering System'.

Please repost your question in the correct blog.

Thanks,

Russ

Ulf said...

Hi! Russ!
Thanks a lot for you help in this friendly way.

You really helped me very much.

To the studio look your pictrues have.

Often I read soft light would be the key. But when I have a cloudy day (which should make very diffuse and soft light) my pictrues mostly become bad. There is no contrast no life no colours. So What is the best outdoors light?
Often I heard when there is cloudy it is the best but not with me.
Direct sunlight again is not good. Than often people say " take the model in the shadow (under a tree or behind a house) and then that's the aim.
If it is too dark use the flash.
So is the flash important for your look, or more the ambient light (special outdoor) and what is the best for getting these hollywood look.
thanks a lot

Russ MacDonald said...

Hi Ulf,

Your question is pretty far off topic for this blog. I am trying to keep this blog focused on TTL Flash Metering.

So, please send me an email at russ@russmacdonaldphotos.com.

Thanks,

Russ

robinbajaj said...

Hi Russ,
Thanks for writing such informative blog posts. I am learning so much from you..I have of a couple of relevant questions *(related to the use of flash in TTL and TTL-BL mode) that I have posted on Nikonians forums - may be you can address that.

http://www.nikonians.org/forums/dcboard.php?az=show_topic&forum=173&topic_id=74567&mesg_id=74567&page=

http://www.nikonians.org/forums/dcboard.php?az=show_topic&forum=173&topic_id=74565&mesg_id=74565&page=

http://www.nikonians.org/forums/dcboard.php?az=show_topic&forum=173&topic_id=74563&mesg_id=74563&page=

Sorry if its not the right way of asking for help from you.

Let me know i can post the question here on your blog too except that I cann't post the pics here (or can i ? )
thanks,
Rooban.

Frank said...

Hi Russ

I also want to thank you for a very informativ blog, and for your posts on Nikonians. It's a great help.

There is one thing, that I dont understand. You wrote in your post:

"The Camera metering system does not take into account that the TTL flash will add to the exposure of the subject. You can prove this by taking a picture, with and without flash, with the shutter set at a fixed amount. Set the camera on S mode and the shutter to 1/80th and let the camera pick the f/stop. The f/ stop that the camera selects is the same whether the flash is on or off. This means that whatever ambient light is reflected from the subject will be further illuminated by the the flash, and if the ambient is already enough to properly expose the subject, the flash will cause overexposure."

I dont get this result. When I turn my SB900 on (camera: D50) the shutterspeed increases often (in aperture mode) and when in shutter priority, the camera goes for a smaller aperture.

The flash is in TTL mode. (not TTL-BL)

In other words it seems to me, that the cameras exposure setting is affected by the flash...

Do you have any comments on that?

Thanks again for a brilliant blog.

Frank

Russ MacDonald said...

Hi Frank,

I wrote this blog based on the D70 and D200 cameras, and for those cameras it works as I described.

However, Nikon has made some changes to the camera 'programs' (P, S, and A) in the newer cameras in an attempt to control the overexposure problem that I described when using regular TTL in bright ambient conditions.

I see this change on my D3. In fact, the program change occurs in every one of the Auto modes, P, S, and A. This does not happen on the D70 and D200.

I believe this camera program change is a predetermined bias being applied whenever the flash is turned ON. This will clearly reduce the ambient contribution to the image by at least a stop - maybe more, and this should greatly help with the overexposure problem I described.

I do not think the change is intelligent, meaning, it is probably using a look-up table to set the change.

I will study this and try to fully understand exactly what it is doing, and then I will add this info to my blog to explain the differences in the newer cameras.

Nikon is going to have to start letting me know when they make changes like this :)

Thanks for pointing it out.

Russ

Frank said...

Aha, I see. That should be a big advantage. Thanks a lot for explaining it, and thanks again for your work.

Best regards

Frank

Frank said...

Hi Russ

I now get up one hour earlier every day to be able to read your posts before going to work... Its a fantastic resource!

However it also generates som questions from my side. (They have reference to your first blog “Nikon Flash two separet metering systems”. For some reason I cant leave a coment there, so I try here instead. Hope thats allright.)


Question:

In an answer to "Bubble" from march 26, you wrote:


"So, in the case that you described, where you changed the shutter from 1/30th to 1/60th, the flash contribution to the image won't change, but the ambient contribution will increase by one stop".

The ambient contribution will increase? I dont get this. Maybe I read it the wrong way, but shouldnt it be the opposite?

Also in the same answer, you wrote:

"...When the ambient light is bright enough to significantly affect the subject, then changes of shutter speed will change the brightness of the subject as well as the background. The background will still be affected more than the subject".

and

"When I run into this situation, I simply increase the flash power to overpower the ambient, so all the assumptions will work again".

and

"This situation normally only causes a problem outdoors in daylight conditions. Then, the daylight is so bright that you cannot increase the flash enough to overpower it, and you have to use a 'fill' flash concept. It is very rare when shooting indoor flash that you cannot increase the flash power sufficiently to overpower the ambient, so that you can once again control the background and subject independently".

My question:

I am a bit confused about the concept of "overpowering the ambient".

In my understanding (and it may verywell be wrong), you "overpower" the ambient by use of a fast shutter. Isn't this what it is all about - turning background or subject dark by using a fast shutter, and after that you can control the brightness of the subject, by using your flash?

Hope you have the time to clarify this for me.

Thanks!

Frank
(Denmark)

Russ MacDonald said...

Hi Frank,

I wrote you an answer yesterday, but I guess it didn't post for some reason.

Anyway, you are right. I mad an error (twice) 1/30th and 1/60th should be reversed.

And when I said I "increase the flash power to overpower the ambient" , I meant to increase the flash power relative to the ambient. Of course, the way you do that is to decrease the ambient. It was a poor choice of words on my part.

Hope that is clearer,

Russ

Frank said...

Allright, thanks a lot for your help.

Daniel said...

I was trying some fill flash in super bright sunlight, manual mode, TTL-BL, but was limited by the shutter speed of 1/250s. Overexposed background. You mentioned that in this case you usually shoot in either S or P modes. In S mode, would you still be limited to the 1/250s max shutter speed? And in P mode, does the camera just bypass the shutter speed also? Also, is S or P modes do you still expose for the background? Thanks Russ for all your help. Daniel

Russ MacDonald said...

Hi Daniel,

The reason I like to use S or P mode when shooting fill is that the camera will close down the aperture to control the exposure.

The 1/250th Flash Sync speed is always enforced unless you use Auto FP High Speed Sync mode. Of course, the problem with that mode is that it reduces the maximum effective power of the flash to less than half of what it is in regular sync.

So, using S or P modes does not change the maximum shutter speed.

Russ

Anonymous said...

I have a nikon 70-300mm VRII lens with my Nikon d700 (and 24-70 lens). I am thinking of getting a fast, constant aperture lens to shoot the events (in low light). Since I cann't afford the Nikon 70-200mm VR or VRII I am considering buying the Nikon's non-VR 80-200 AF-D lens. The only concern a friend shared is the ability to hand-hold without VR at around 1/40-1/50th sec shutter speeds during event photography.
Now I am thinking that since in the indoor events (after following your blog's advice) I will be using TTL flash to freeze the foreground anyway, the only motion blur that one might witness is in the background which can be (in most cases) safely ignored or controlled by underexposing the background deliberately by a faster shutter speed.

I thought I should confirm this assumption with you before going ahead with my 80-200 AF-D purchase. What do you suggest - being a field professional and a wedding photographer. I am not debating whether VR lens is good or not. I am debating whether one can do without it (using a sharp AF and sharp (f2.8 wise) lens that just misses the VR). of course I don't have any special hand-holding capabilities that others don't.

Thanks in advance,
Regards,
-Rooban.

Howard said...

"On a D200 camera you can easily demonstrate this problem ... Set the camera on S mode and the shutter to 1/80th and let the camera pick the f/stop. The f/ stop that the camera selects is the same whether the flash is on or off."

This is not what I see with the D200 and SB-600. The aperture changes when the flash is turned on, although it can be a minimal change, such as from f8 to f9. More importantly, my exposures are not overexposed under the conditions you describe. Either I am not understanding your description or Nikon has somehow upgraded its flash functioning (with a firmware update?).

Russ MacDonald said...

Hi Rooban,

The general rule on handholding a lens is that you should use a shutter speed that is 1/focal length (FX). This means if you are using a 200mm setting on your lens, that is equivalent to a 300mm on FX so you should use a minimum shutter speed of 1/300th sec.

However, you are exactly right. If you underexpose the ambient and let the flash supply the exposure, that's like using a 1/800th shutter speed or higher, and the flash will freeze the action. You are also right that any ambient in the background will be blurred, but it ususally doesn't matter.

The 80-200mm Nikon lens is a great lens for using your flash. Just remember to always underexpose the ambient or you will get blur.

Russ

Anonymous said...

Hi Russ,
Thanks for your response. My concern is whether I will be able to make flash the primary light source when shooting at 200mm (on full frame camera). I do understand I could get a of max distance range around 66 ft (I think so..) with my SB-900 and at 200mm I might well be just at that distance.. I am just thinking if it will be hard to make flash primary light source when shooting from such large distance.. and when shooting portraits or isolating subjects (like a single person or a couple) from the background.. i think it looks nicer if the ambient is taken in as-is vs when underexposed.. what do you think ?

thx in advance,
Rooban

jshelbourn said...

Hi Russ I have read with interest( a lot of Interest) your articles of Nikon TTL, BUT I need help. I have a D200 and want to do more portrait shots ( mainly Black & White) I cannot find anywhere a book or info in depth ( simple stuff) about all you have to do.
ie change setting up wifi off camera SB600 & 800s and the setup in camera for manual. I tried to take manual shots and found that I could not get the speed up past 1/60 for a long long time.
so, it would be useful to have a step by step run at what to do. I find that the shops are to bright and I am not getting shadow I want. Can you help with a book or video or something please. I'm in Australia

Anonymous said...

Hi Russ I have read with interest( a lot of Interest) your articles of Nikon TTL, BUT I need help. I have a D200 and want to do more portrait shots ( mainly Black & White) I cannot find anywhere a book or info in depth ( simple stuff) about all you have to do.
ie change setting up wifi off camera SB600 & 800s and the setup in camera for manual. I tried to take manual shots and found that I could not get the speed up past 1/60 for a long long time.
so, it would be useful to have a step by step run at what to do. I find that the shops are to bright and I am not getting shadow I want. Can you help with a book or video or something please. I'm in Australia

Russ MacDonald said...

Hi jshelbourn,

Have you tried Mike Hagen's book 'The Nikon Creative Lighting System: Using the SB-600, SB-800, SB-900, and R1C1 Flashes '?

I think it has the details you need.

Regards,

Russ

jshelbourn said...

Hi Russ,

Thanks so much I will try and get it. I love the posts I have found them very informative. Will you be doing any posts on other forms of shooting?

But this great thanks again

JS

Russ MacDonald said...

Hi again jshelbourn,

Thanks for the feedback.

I do this for a hobby and to try my best to help others. What I write about, and how often I publish, depends on how much time I have.

Also, I try to keep my comments generic so they apply to all Nikon cameras and speedlights. I don't think I would ever be interested in something as specific as how to set up a D200 to use with a remote SB600, for instance.

However, I would be glad to help you by email if you will send your questions to me. Just get my email off my profile or website.

Russ

Anonymous said...

Hello. I am using D700 coupled with SU-800 commander and a SB-900 in remote mode. I mostly shoot in M(camera) in this setup. You've written before, in regards to SB-800(but I hope it's also valid for SB-900), that when in TTL, the front of the flash(red window) does not have to be aimed at the subject. This is good news to me because I often have to rotate the whole unit to get that IR sensor(which unfortunately is only on one side of the flash) point my way, which often results the front pointing away from the subject. My question is, in that case, how(by what) is the pre-flash measured to calculate the proper power?
Thank you,
David

Russ MacDonald said...

Hi David,

Yes, the SB-800 and SB-900 function identically (except for the more extensive zooming capability of the SB-900).

The reason that the flash body doesn't have to be aimed at the subject is because it is a TTL (Through The Lens) system.

The measurement of the monitor preflash is actually done in the camera by the flash computer software.

Then the camera calculates the correct power for the flash and sends it to the flash, either through the hot shoe or via the IR pulses sent to the Remote flashes.

So, for the Remote flash, all the Commands and Power Data pass through its IR window.

Hope that helps,

Russ

Anonymous said...

I understand now-many thanks! My mistake was to assume that the flash computer/software is located inside the sb 900.
David.

Anonymous said...

BTW, I've read somewhere that if you put a diffusion dome onto su-800 commander you'll increase the angle range for communication with remote units. I put this claim to the test and it proved to be a GREAT improvement! By simply attaching the plastic diffusion dome(the one that comes with sb900) with a rubber band onto the su-800 I was able to place the remote unit about 30 degrees behind me (12' away 6' high) and trigger the flash. Normally, you can't even do 90 degrees when out in the open. Sorry for this off-topic post but maybe it'll help somebody. David.

Russ MacDonald said...

Hi again David,

That's a great idea about taping the diffuser dome to the SU-800. I don't own an SB-900, so I will have to figure out a way to try that with the dome from my SB-800.

Thanks!

Russ

Anonymous said...

Dear Russ,

I am dumbfounded by the kindness and generosity demonstrated by your patient replies to the many questions posted to your site. And this on top of your excellent posts.

Please forgive the stupidity of this question. You have mentioned frequently the difference between on-camera and off-camera use of speedlights. I suddenly realised that I'm confused by what "on-camera" and "off-camera" actually means. I'm assuming that a speedlight mounted in the camera's accessory shoe is "on-camera". What if the speedlight is mounted on a bracket attached to the camera and connected to the accessory shoe with an SC-17, SC-28 or SC29 cable? Is it "on-camera" or "off-camera"? What if the speedlight is clamped to a nearby pole but connected to the accessory shoe with a cable? Physically it's "off-camera", but...

Does "on-camera" and "off-camera" refer to the physical location of the speedlight, or the nature of its connection - wired or wireless - to the camera?

Again, please excuse the stupidity of this question. And thanks again for sharing your knowledge so lucidly.

D.

Anonymous said...

Russ,

Thanks for a very clear and useful article. I just had a real "Eureka" moment!

Regards,

Michael

Russ MacDonald said...

Hi Michael,

I'm glad you found my article useful. Let me know if you have questions.

Russ

Russ MacDonald said...

To D:

I was looking back through the questions and noticed that I neglected to respond to you.

You asked what I mean by the terms 'on-camera' and 'off-camera'.

I haven't gone back and checked every entry in my blog, but in general, when I say 'on-camera', I mean that the flash is mounted in the hot shoe.

When I say 'off-camera' I mean the flash is NOT mounted in the hot shoe.

Now, if the flash is 'off-camera' but it is connected to the camera by one of the flash cables (like the SC-29), the camera will 'think' the flash is still mounted in the hot shoe, so it will performs all the same computer algorythms. This works correctly in regular TTL mode, but in TTL-BL mode it can cause it set the flash power incorrectly. This is because the distance that the lens reports is often the main input to the flash power calculation. So, if the flash is positioned at a different distance to the subject than the camera, the flash computer will set an incorrect flash power.

Hope this answers your question.

Russ

jude said...

snaps are reality..and wedding photographers done a great job to have this.

whanauli said...

Hi Russel,
Quote: I don't know if this is the appropriate forum but i really need your help.

I take pictures of dance recitals (street and cheer dance)and i almost always end up with either dark, under and overexposed, blurry pictures.

If the use of flash is allowed, could you please recommend appropriate camera settings which i can adopt so my pictures wouldn't end up bad.

I'm using a nikon d50 and a nikon sb800. The lighting at the venue is always changing. I would be about 15-20ft from the stage and i use a tamron 28-75mm f/2.8 lens.

Should i go manual or program mode? TTL or manual in the flash? Use a stofen or not? Auto ISO or not?

I know i'm asking a lot but i've been taking pics of events (dances and concerts) for so long that i feel i haven't been learning from previous shoots though i've been applying what i read from your articles and others. Help please.
September 22, 2008 5:11 PM

Can you share the solution or advice of this problem please as I am new to photography and CLS.
This blog is the best one I read. Thanks for your good work.
Regards Whanauli.
My email: whanauli@gmail.com

Anonymous said...

Russ,

Thanks for your very informative web site. I thought you might be interested in this link which I believe is one of the major patents for the CLS:

http://www.google.com/patents?id=1bBQAQAAEBAJ&zoom=4&dq=nikon%20creative%20lighting%20system&pg=PA1#v=onepage&q&f=false

Apparently Hidehiko Aoyagi was its inventor.

Russ MacDonald said...

Hi Anonymous (wish you would sign your name when you post):

No, this invention is not one of the CLS patents. The CLS patents were all filed in the 1990's. This one was filed in 2008.

This patent filing is for a new invention that is not in production yet.

It involves adjusting the temperature of the light in both the monitor preflash and the main flash, to get more accurate flash power settings. Currently, the flash power is set by a monitor flash pulse and main flash pulse that are both 5500K. This causes problems. A single temperature light will reflect different amounts from different color surfaces leading to flash power deviations. You would like your flash to reflect the same amount from whatever color surface it hits, so the power will always be set the same. This invention will theoretically correct this problem.

KVS Setty said...

Very very informative content,
explaining a very complex subject in very simple and precise form. thanks a lot for the great job.

I have a question, if we move the flash off the camera, you said we loose TTL- BL metering , that means the off camera metering is standard TTL right? but in DPanswers.com in an article they say quite a opposite way.
see this link:
http://dpanswers.com/content/nikon_flash_use02.php

Under the heading: 3. Speedlight Flash Modes

Please help me to know the correct metering method in off camera flash method

Russ MacDonald said...

Hi KVS Setty,

I read the article you referenced, and all I can say is that it is not correct. When a flash is in Remote mode it cannot work in TTL-BL mode.

It's easy to understand why TTL-BL cannot occur when the flash is set to Remote. For TTL-BL to operate, the flash must be located at the same location as the camera (or close). This is so the ambient measurement that the camera makes will send the correct value of flash power to the speedlight. If the speedlight is positioned off to one side, the power sent to it will not be right.

So, when the flash is in Remote mode, the only mode it can function in is regular TTL.

This same issue comes up when using an SC-28 or SC-29 flash cable to attach the flash to a bracket or even hand-hold it. This cable will allow the flash to be put in TTL-BL mode, but you have to be very careful to keep the flash close to the camera or the power it fires at will not be right.

The TTL-BL system depends on the flash being at or near the location of the camera.

Once you understand this, you can see that there it impossible for the system to fucntion in TTL-BL mode when the flash is in Remote mode.

The rest of the article appears correct, but I did not study it too closely for all the details.

Russ

Anonymous said...

Hi Russ, after a while i was studying your blogs again, i must say everytime i find new information. A very helpful summary.
What puzzles me is the influence of ISO: The flash computer adjusts the flash power according to the reflected light. So, if you modify the aperture, it adjusts automatically and the subject remains at the same brightness. The same correction occurs when bouncing the flash. What about ISO change, how detects the computer that he has to reduce/increase power? I thought the ISO values lead to higher/lower amplification of the pixel values on the sensor which is way after the brightness measurement takes place in the camera.
Regards, Wolfgang

Russ MacDonald said...

Hi Wolfgang,

The flash computer adjusts for changes in ISO just exactly like changes in aperture. If the ISO is increased, the flash power decreases and vice versa. The flash computer will always keep the brightness of the subject the same with changes in ISO.

Also, you can change the maximum range of the flash by changing ISO. If you increase ISO you increase maximum flash range and vice versa.

Also, you can change the number of flashes per charge. If you increase ISO, the flash will fire at a lower power, and you will increase the number of flashes per charge and vice versa.

However, when you change ISO, you also change the ambient contribution to the image, just like with the aperture, so you have to adjust the shutter an equal amount in the opposite direction to keep the ambient the same when changing ISO. The shutter does not affect the flash [power, since the entire flash occurs within the shutter 'open' period of time. For instance, if you are using 1/80th shutter and ISO 400, and you increase ISO to one stop to 800, you have to also increase the shutter by one stop to 1/160th to maintain the same ambient contribution.

Hope that answers your question.

Russ

Anonymous said...

Hi Russ, thanks for the speedy reply. I understand, but i am after some more technical explanation (hope it is not too detailed): When the aperture changes, the lense widens or closes, more or less light comes in and the metering sensor detects it and informs the flash computer. Same when amount of light changes due to bouncing on the ceiling or so. But when ISO changes what happens in the camera that makes the flash computer aware that he has to adjust? Amount of light that comes in and is measured is not influenced by ISO change or am i wrong? Regards, Wolfgang

Russ MacDonald said...

Hi Wolfgang,

The ISO value is sent to the flash computer.

Of course, the flash computer is located in the camera. You do realize that, right?

Russ

Anonymous said...

Hi Russ, everything clear now. Thank you.
Greetings, Wolfgang

James Clark said...

Russ,

Thank you so much for the great advice and explanations! I have been using your techniques with great success except for one problem. I consistently get underexposed (about 1 to 2 stops) pictures when using my SB-700 off camera in remote mode. It doesn't matter what the subject is or what color.

Here is a typical shooting situation: Nikon D7000 and SB-700. Indoor with typical light, Manual F/4 and 1/80th to 1/125th, ISO 400, iTTL, AF-S, Spot Meter and Focus. I use the SB-700 on camera and the picture looks great. I take it off and use the D7000 flash as commander and SB-700 as remote. I hold the 700 right above the camera so the distance to subject stays the same. I focus on the same subject and the exact same spot and I get an underexposed image, every time. It doesn't matter what the subject is. If I use + 1.0 ev it looks about right.

I would greatly appreciate any advice to correct this. Thank you, James

Anonymous said...

I can´t stop reading this, these expanations are so great !

Anonymous said...

Russ,

There is some misinformation on this page.

You say that the Camera metering sensor and the TTL flash metering sensor are the same, but they are not the same! They are located far apart.

Look at the diagrams below. Self-explanatory

http://www.mir.com.my/rb/photography/hardwares/classics/NikonF5/metering/
http://nikonasia-en.custhelp.com/app/answers/detail/a_id/4492

Their capabilities are also different. The camera (ambient) metering sensor can detect color (RGB). The flash metering sensor is color-blind - only shades of gray.