Thursday, March 6, 2008

9. TTL and TTL-BL Study

After several discussions on Nikonians Forums concerning how TTL and TTL-BL work with respect to off-center subjects and distance, I thought I'd do a study to really find out for sure. Until now, all my comments on this topic have been based on what I've seen in my wedding photography. Now, I can say for sure exactly what is taking place.

This is more of a practical study than a scientific one. I have mostly been looking for effects, and not setting up control shots for precise measurements. I'm interested in how to use this knowledge to take better pictures, not for a review of the system for a specification.

The first thing I wanted to experiment with was the issue of how TTL and TTL-BL differ when shooting a light subject on a dark background at the same distance.

All the images in this study were shot with the following:
D200 Camera on a tripod
SB800 flash in the hot shoe with the head aimed forward
Nikkor 17-55mm f/2.8 AF-S DX lens
Matrix metering
Auto WB
f/2.8 and 1/60th sec unless noted otherwise
The location was indoors during daytime with blinds closed and a low intensity incandescent ceiling lamp on
AF-S Focusing unless noted otherwise

How Subject Position Affects Flash Power

In this set of tests I used a dark blue bathroom towel as a background and I placed a round white paper targets (the 'subject') at various places within the frame to see how the power of the flash would adjust. The reason I used a bathroom towel was that the texture of the towel provided a good AF target and I consistently got sharp and solid focus.

Image 1 Reference image, no flash , f/2.8, 1/1.6 sec

In Image 1, I used available light only, just as a reference. Notice the color of the bright 'blue' towel has shifted due to the Auto WB, which tried to make it 18% gray. The histogram shows a narrow peak representing the narrow range of brightness values for this image.

Image 2. Flash TTL, f/2.8, 1/60th sec

Image 2 is also for reference. It also hows how the Auto WB gets the color right when flash is used. The histogram shows a similar pixel distribution as in Image 1.

Image 3. Flash Off, f/2.8, 1/60th second

Image 3 is another reference image using the same f/ stop and shutter that will be used for the rest of the images in this study. Notice that the ambient is so low that will not be a factor in any of the remaining images in this study.

Image 4. White disk top right corner, Flash TTL

Image 4 shows the effect on the histogram of introducing a white disk of about 3 inches in diameter into the upper right corner of the frame. If you compare this to image 2 you can see that the background (the towel) has gotten slightly darker as a result of the flash metering system 'seeing' the white disk. Now, carefully look at the histogram, and you can see that the main hump of pixels representing the dark blue towel has moved left, and there is a small hump of pixels bunched against the very right edge. This small hump of pixels is the white disk, and its presence as a highly reflective subject has forced the flash power to decrease, which is why the background (the towel) got darker. The fact that the pixels for the white disk are climbing the right edge means that the flash has blown them out. Those pixels are overexposed and if there were any detail in such an area, it would be unrecoverable.

Image 5. Disk in upper right corner, Flash TTL-BL

Image 5 shows the effect of using the flash in TTL-BL mode. Comparing to Image 4, which used TTL, we see that TTL-BL sees the disk and tries to match it to the background. The result is actually quite good due to the fact that TTL-BL also uses distance information from the lens to set the power. This all has the effect of making the entire image much darker and the background is nearly black. The pixel hump representing the background has moved nearly to the left edge of the histogram, while the smaller pixel hump representing the disk has also moved left so it is no longer climbing the right edge. There are no blown out pixels in this image and it would be usable.

I am skipping Image 6 and 7 where I moved the disk to the bottom left corner, because the brightness and histogram distribution were essentially identical to Images 4 and 5.

Image 8. Disk centered, Flash TTL

In Image 8, I moved the disk to the center of the frame, and used the flash in TTL mode. You can see that now the flash metering system is more greatly affected by the disk more than it was in Image 4 when the disk was in the corner, and it decreased the flash power further. You can tell by the left shift of the background hump (towel pixels), and if you look closely, you will see that there is one thin line of pixels climbing the right edge indicating that the disk is totally blown out. So, even with the substantial darkening by moving the disk to the center, it is still not enough to prevent the disk from being totally blown out. This is because the disk is smaller than the center weighted measurement area, which allows a considerable portion of the dark towel to influence the metering. For proper monitor preflash measurements, the subject must be centered and occupy 18% of the frame. In this case it occupies only 7% of the frame, so lots of the dark towel is getting involved.

Image 9. Disk in center, TTL-BL

In Image 9, I switched the flash to TTL-BL and you can see that once again, the flash metering system tried to reduce the brightness of the subject (the disk) to match the brightness background (the towel). This moved the histogram to the left so that it looks essentially identical to Image 6, and indeed it should, because both were balanced to the dark towel. The fact that the disk was in the center made no difference. To state this another way, TTL-BL works differently when the background is darker than the subject. This is an area that is the subject of much research, and newer cameras than my D200 will likely deal with the dark background problem much better. It is my understanding that they are working on a way to switch to regular TTL when the background is dark. If this works out, it may obsolete regular TTL.

Focal Distance Tests Using a DX lens

Image 10. Disk in center, Flash TTL-BL, focus distance set manually to infinity

In Image 10, I am studying the effect that focal distance has on flash power.

I used TTL-BL and manually refocused the lens to infinity, and you can see the dramatic effect. Obviously, the image is no longer sharp, because it is way out of focus. The lens is a DX lens, which reports its focal distance, and it told the flash metering system that the subject was at infinity, so the flash increased to a very high power for the shot. I have no way to tell exactly what power the flash chose, but you can see that the histogram was shifted so far to the right that the background pixel hump is now in the center, and if you look really close, you can see the subject pixels are climbing the right edge compressed into a thin line, totally blown out.

Image 11. Disk in the center, TTL, focused at infinity

In Image 11 I switched the flash to TTL and refocused the lens to infinity. If you compare Image 11 to Image 8 you will see that they are essentially identical in brightness (not image sharpness), which indicates that the flash power was not affected by changing the the focal length of the lens. To state this another way, distance is included in the flash power calculation when using TTL-BL but not when using TTL.


1. In TTL flash mode, the subject reflects more light back to the flash metering system when the subject is centered. This is consistent with the comments I made in my earlier blogs that the flash monitors the center weighted area of the frame. I have shown through this study that the flash power is influenced more strongly by the subject when it is centered, but it is also influenced to a lesser degree all the way to the edge of the frame.

2. It is totally clear that in TTL mode the subject will often be overexposed if the flash ev is not reduced. I have found in my own wedding and events work that the flash works best when the flash compensation is set to about -1.0 ev for indoors.

3. TTL mode is NOT influenced by the distance that the lens reports.

4. TTL-BL mode is heavily influenced by the distance that the lens reports. I did not do a full study here, so I don't know exactly how much it increased, but by rough comparison with the flash in manual mode it appears to have increased to at least 1/2 power if not higher. This further implies that whenever we use TTL-BL mode we must make sure the distance from the camera to the subject is equal to the distance of the flash to the subject. In other words, don't use TTL-BL with the flash on a flash cable unless it is kept at the same distance to the subject as the camera.

5. It is totally clear that when the background is darker than the subject TTL-BL reduces the power of the flash in an attempt to balance the subject to the background. This works OK much of the time, but it is still best to use TTL indoors, where the subject is usually brighter than the background. However, I expect this conclusion to change as the newer versions of TTL-BL emerge on newer cameras.


Justis Photography said...

Well put and as always, thank you for making these flash tutorials much easier to understand. I normally only use TTL-BL outdoors in harsh sunlight. The rest of the time I am in Manual mode.

Anonymous said...

Thank you very much for these wonderful tutorials!!
You just save me from falling into a mental hospital!! :)

Anonymous said...

Fantastic exploration of the Nikon iTTL system and how it works. Easily the best explanation and understanding of exactly how the system works, and therefore how it should react in any given situation if one understands what is going on.

It would appear to me that Nikon should offer you a PR position for their iTTL flash system and write a complete manual for its use. I would guess that you probably know better than even Nikon how it works.

Keep it up !!

Russ MacDonald said...

Bubba59, Lance (Justis), Ale,

Thanks for the great feedback.

Nikon makes a great flash system, but they don't do a very good job explaining EXACTLY how it works.

In fact, I haven't found any book or publication that fully explains all aspects. You need all the missing information to use the Nikon flash to it's fullest potential.

I did this work originally for my own purposes, but it is now fun to distribute the results so everyone can benefit.

Thanks again,


Hyorrana Mikaelly said...

Eu queria que você, se pudesse, me explicar melhor, sobre o funcionamento dos modos, não somente TTL e TTL-BL, sou iniciante em fotografia, mas estou um pouco crua em relação a iluminação deste tipo.

Russ MacDonald said...


I'm sorry but I do not understand. I used a web tranlation from Portugese to English, but I am still not sure what you are asking.

If you are asking for more basic information on TTL and TTL-BL, I suggest you read my other posts on my blog.


Anonymous said...


Thanks for this. It's excellent and much clearer than the Nikon manuals. I really appreciate it. One question: You mentioned that the distance from flash to subject should be the same as camera to subject when TTL-BL is used. Does this mean then that if I were to use several speedlights remotely, I would need to be in TTL only?

Russ MacDonald said...


TTL-BL is not available when the flash is used remotely. As soon as it is removed from the camera hot shoe, TTL-BL is not available. Also, when used in Remote mode, the only selection on the Commander screen is for TTL.

However, if you use a hot shoe cable like the Nikon SC-29 that allows you to move the flash off-camera, then TTL-BL is available, and that's when you can get into trouble if you don't keep the distance from the camera to the subject the same as the distance from the flash to the subject. When using a cable like this, the camera thinks the flash is still mounted on-camera.


Anonymous said...

Thank you for a wonderful blog. I have read your tutorials end to end and am trying to get them fully ingrained into my head. You're doing a wonderful job here!

With regards to the off-camera cable and the need to ensure the flash-to-subject distance is the same as the camera-to-subject distance, there's an additional point here to remember: When the flash is hot shoe mounted (or indeed connected using the cable), the flash will zoom itself to match the lens focal length (unless the zoom has been switched off in the flash settings). So if one moves the flash onto a camera bracket to the left, say, and then focuses tightly on the subject, the flash will do its best to focus the light tightly on an area to the left of where the subject is. Also not a good recipe for perfect photos!

Russ MacDonald said...


Excellent point!

I never thought about how the zoom function on the flash could cause a problem with the flash on a bracket.

I will definitely update my blog to add a discussion about that.

Thanks for your comment,


Anonymous said...


Thank you for taking the time and effort to put together such a helpful blog. You have been able to clrify so many issues that I have spent many hours trying to figure out. I am anxious to put this newly gained knowledge to the test. Once again thank you!!!

Russ MacDonald said...


You are most welcome.

Thanks so much for the encouraging feedback.


Anonymous said...


Thanks for your blog.

It would be great if you could collect all your articles and produce a small .pdf file we could print and carry with us.

Russ MacDonald said...

Hi Jaime,

I will consider your suggestion.

However, I am afraid it would make changes to my work much more difficult, since I constantly refine and update my blogs, based on feedback and additional questions. Over time, my blogs have become more accurate and easier to read.

Thanks for the suggestion,


Kimrock said...

You've mentioned that you put flash ev to -1 while shooting indoor with Sb-800, does the same applies to SB-600?

Russ MacDonald said...

Hi Kimrock,

The actual setting you use on your flash may not be -1.0 ev. Different camera/flashes function slightly differently.

I always underexpose just slightly if I don't have time to check the histogram and blinking highlights screens. This is to prevent blowing out a face. That's the worst thing that can happen, and if you are shooting critical shots that have to be right, then you absolutely must avoid blowing out a face.

In post processing you can always recover a slightly underexposed image but there is nothing you can to to recover a blown out face.

So, with the SB-600, which I also own and use, I normally turn it down only -0.3 ev, because I find that it fires not quite as 'hot' as the SB-800's.

Now, if you are trying for a perfect image, and you have time to check the histogram and blinking highlights, then push the brightest pixels close to the right edge. That will give you the lowest noise and moise detail possible. Then, if you push it too far (and blow out a face), you adjust and shoot another shot.

Hope that helps,


Kimrock said...

Thank You for your reply. That helped me a lot. This is great blog site we've ever wanted. I think Nikon should thank u for this.
I'm a student and an amature photographer and using D40x and SB-600(to stay within my budget).
I'm finding my pictures overexposed due to lack of FV lock in my camera. I cannot make good exposure with the shots like two people shaking hands, a person giving a certificate to another person(using TTL, matrix metering, 1/80sec, f/5.6, iso 200). Such conditions don't provide me enough time to check histogram. iTTL-BL seems to work for off centerd images but it's unpredictiable under different kinds of ambient light. Could u please suggest me any solution except changing camera and using flash in manual mode(i'm very bad in judging a distance).

When a person is wearing highly reflective materials like metal chains, metal earings or white clothes; is it reliable to judge an exposure by looking at a histogram as you have said before?

Russ MacDonald said...

Hi again Kimrock,

First, for any indoor shot in artificial light you should always use TTL - not TTL-BL.

Then, the problem with taking a flash picture of two people shaking hands is that you are aiming between them, and the flash metering is then looking at the background. It see the return flash weak from the background, so it increases the flash power.

The solution is to always widen your focal length (zoom back a little) until you can place one of the two people in the center ofd the frame. Then, you recenter the picture correctly in post processing by cropping.

The other thing you can do is always reduce the flash to about -0.7 ev. Then, you restore the correct brightness in post processing. This is what I always do to avoid blowing out faces.

When a person is wearing highly reflective jewelry, it will make 'specular highlight', but the flash system is usually smart enough that it doesn't reduce the flash power. You normally just let those speciular highlights blow out and it looks fine.

Now, if a person is wearing white clothes, I always immediately turn up the flash to +1.0 ev if the person will be filling the frame. If the person will not be filling the frame, then 0 ev usually works best.

You do exactly the opposite if the person is wearing black. You turn down the flash or it will overexpose.

Again, everything you do is in relation to where you place the subject in the frame. The flash always meters the center weighted area of the frame to set its power.

Hope that helps,


Kimrock said...

Thank you very much for your support.

I'm shooting with your tips now.

Thanks again!

Anonymous said...

In your study for flash with distance did you use a G or D lenses? Or do these lenses offer no added benefit for matrix fill?

Russ MacDonald said...

To Anonymous,

I used a Nikon 17-55 AF-S f/2.8 G DX lens for my tests.

However, there is no real difference in flash photography between the G and the D lens. Both report distance info to the flash computer.

Of course, as I mentioned in my blog, distance information is only used in TTL-BL mode. Furthermore, it is only used when the flash head is pointed straight forward.

Distance information from the lens is never used when in TTL mode.


Anonymous said...

Thanks for taking the time to create and share this information. As a new owner of a Nikon D700 and SB900 flash I have really been taxing my brain reading the manual and trying to understand. Your information has cleared up many questions for me.


Russ MacDonald said...

Hi Jeff,

Thanks for the nice feedback!

If anything is unclear please post questions.


Anonymous said...

it's written in the manual that focus distance matters for the metering system.

why do you think it's your discovery?

Russ MacDonald said...

To Anonymous,

When I began to study this, I thought that every D and G lens would send distance to the flash, and that information would be incorporated into the flash power calculations.

However, I discovered that the distance information reported by the lens is ONLY used in flash power calculations when in TTL-BL mode.

More surprising to me was that this distance information is NOT USED in TTL mode.

I have not read the SB-900 manual, and it may be explained there, but it is not explained in the SB-800 manual.

Did I understand your question correctly?


John said...


Can you give some helpful hints on shooting a bride (dressed in white) and a groom in a black tux?

Thank you.


Russ MacDonald said...

Hi John,

Any time you shoot a person, you have to decide first how tight the shot will be. If you zoom in for a tight head & shoulders shot, then the clothing color and reflectivity doesn't affect the flash as much.

However, if you back up and include a significant amount of the bride's white dress or the grooms black suit, it will have a major impact on the exposure.

If there is a lot of the white dress in the picture, then the flash will see the bright reflection and decrease its power significantly. In this situation, I usually increase the flash by about 0.7 ev over what I have been using. I also check the histogram and flashing highlights screens to be sure the added exposure doesn't blow out the bride's face.

If there will be a lot of the black suit in the picture, then the flash will think it is not bright enough and increase its power. This is the absolute worst situation if you don't catch it, because it will definitely blow out the groom's face. You can fix underexposre in post processing, but there is nothing you can do that will fix a blown face. I always decrease the flash by 1 ev when I shoot the groom to include the black suit. Then, I also check the histogram and flashing highlights screens.

Hope that helps,


John said...

Yes...very helpful. More of a problem for me is when I'm shooting both of them together. That's where I begin to hesitate. I guess "underexpose" is better that "overexpose".

BTW - I've read several of your blogs here related to ttl / ttl bl. They've been really helpful to me understanding why my flash shots are not consistent. I believe my future shots will be much better with the information you've given shared.

If you write the book...please allow me to buy the first one (autographed please).



Russ MacDonald said...

Hi again John,

Whoa! A book is way too much like work! I enjoy just writing this blog and helping people learn more about photography - the flash in particular, since Nikon didn't really tell us all the details about their system.

Now, on to your question: shooting the B&G together is simple as long as you keep as much white in the shot from the bride's gown as from the groom's suit. If the dress is huge, then you may want to increase the flash ~ 0.3 ev, but I usually just brighten it in post processing.

I always make sure to underexpose rather than overexpose.


John said...

Hope you don't mind...just one more thought to run by you...

I've recently bought a sekonic l-358 light meter and I'm starting to do more incident metering. So far I like what I see. I've often thought the built-in meter can be fooled since it's a "reflective" meter. Incident metering appears to be more accurate as I continue to experiment with it. So it would seem with a more accurate metering of a bright background (using my sekonic meter) then setting my d300 to that then setting my sb600 to ttl/bl/fp...well my pictures seem to be even better.

I didn't see much on your blogs related to incident metering vs. reflective metering.

Have you experimented with incident metering vs. the in-camera reflective meter?

Thank you again.


Russ MacDonald said...

Hi again John,

You are again right!

In the past I used my incident meter quite a bit, and it does do a much better job than reflective metering. However, the in-camera metering works so well, I find myself using the incident meter less and less. A lot of it has to do with learning how to compensate the flash by just looking at the subject.

Also, the incident meters are fairly expensive, so my blog is mostly for people who use only the in-camera meter.

But you are right. Incident meters measure the light falling on the subject, so they are never fooled by the reflectivity or color of the subject. This makes them more accurate.

I have found, though, that for most shots, it is faster and just as good to use the in-camera meter and adjust the exposure based on what I see in the histogram and blinking highlights.


Abe said...

Hi Russ,
Reading this enlightening section on Distance/Background/etc. brought up these questions, doubts.

>>when the background is darker than the subject. It reduces the power of the flash drastically in an attempt to balance the subject to the background

Why fire at all?

Is there anyway to see the distance of focus point, does it show in any of the exif or meta data or in the camera (D300)?

when my sb-800 is mounted on camera, and I change the flash ev by pressing the button on the left of the popup does this change the ev of the sb-800 or must I change it on the back of the sb-800? When I tried this I found that changing this ev changes the distance range on the back panel of the sb-800 but that the ev on that panel does not change.

Another curiosity is this, changing the ISO setting obviously changes the distance range of the sb-800. But it seems that this is only until 66' after which no matter how much you increase the ISO it only changes the min but the max goes not over 66'. Have you noticed this? Or is this due to the lens I currently have on the camera, sigma 15-30?

I have also noticed

Russ MacDonald said...

Hi again Abe,

I guess the flash doesn't completely shut off because it's turned on. The thought is that if you have it turned on, you want it to flash, so it simply flashes at its lowest level.

I don't think there is any way to see the distance that was reported by the lens when the shot was taken - at least no way with any of the software we commonly use.

The flash ec button on the camera does exactly the same thing as the flash ec button on the back of the flash. And they add. I quit using the button on the camera after I forgot it a few times and couldn't figure out why my flash pictures were so far wrong. Now, I leave the camera flash ec button at 0.0 all the time.

Watch for that range readout on the back of the flash. It is way off much of the time, especially with high ISOs and when you put the flash in FP mode.

That readout is not used by the flash for any purpose. It is only for the photographer, and then it is not accurate. I don't use that readout for any purpose.


Abe said...

Thanks for your reply.
I did some checking and found that at least this program PhotoME will report the distance. I'm told there are others too.

John Meyer said...

Great blog!

Anonymous said...

I hate to take advantage of a new found (hopefully) friend, but I'm really struggling with this flash stuff and was hoping you would help.

I took several pics of my daughter tonight at sundown and tried several shots using the Gary Phong LS II, and could not get a good shot.

Would it be presumptuous to ask you and the group to look at them?

I reduced the size down to about 1500x1500 so they wouldn't be too large.

Rick Hollingsworth

Russ MacDonald said...


I'd be glad to take a look at your pictures, but I don't see them anywhere.

Please email them to me at


Ken said...

Hi Riss,
Do you still maintain this blog?
I was wondering if bouncing the external flash should change any of the results you demonstrated on this page.