Friday, March 14, 2008

10. Auto FP High Speed Sync Explained

One of the most confusing aspects of the Nikon flash system is this thing called 'Auto FP High Speed Sync'. This blog will hopefully clear up the mystery about this mode.

Before you can understand FP High Speed Sync, you have to understand what FP means and how the Normal Flash Sync works.

FP stands for Focal Plane and it refers to the type of shutter used in most modern DSLR cameras. A focal plane shutter is actually two precisely timed curtains positioned between the lens and the sensor that can either block light from hitting the sensor or allow light to hit the sensor. The reason there are two shutter curtains is to be able to get much higher effective shutter speeds.

It is important to understand is that these curtains open and close in exactly the same amount of time. So the the shutter speed is set by timing between the start of the first curtain opening and the start of the second curtain closing.

Notice that the entire sensor will be open to the light at every shutter speed up to the speed of the curtain movement itself. This is the maximum normal Flash Sync Speed. To say this another way; at all speeds up to the maximum normal Flash Sync Speed, the first curtain completely opens before the second curtain begins to close. At any shutter speed higher than this, the second curtain will begin closing before the first curtain gets fully open, thus never exposing the entire sensor at any one time. At really high shutter speeds, this results in very narrow 'slit' of light that travels across the sensor.

These two curtains travel vertically across the opening from the top to the bottom, and this in itself causes some strange effects when using high shutter speeds (small slit). If you shoot something that moves horizontally really fast like a race car from the side, you can sometimes see that the wheels seem to lean forward a bit because the top part of the wheel was exposed after the bottom part and the top moved forward a little bit as the slit moved from bottom to top. Of course, the image is inverted on the focal plane, exposing from the bottom to the top of the image, and that's why race car wheels lean forward even though the shutter that moves from top to bottom. Also, this effect only occurs if you don't pan with the car. You can totally eliminate the forward leaning effect by panning.

As I mentioned, a focal plane shutter mechanism moves the curtains at a very precise speed. This speed is determined during manufacturing of the mechanism and is governed mostly by how recently the shutter was designed. In older 35mm cameras, this speed was 1/60th second, but with time, shutters got faster and faster, and in the new D300 this speed is 1/320th sec. And those race car wheels lean much further forward with an older 1/60th shutter than with the D300 1/320th shutter.

Once you understand how the shutter works, you can begin to see what needs to be done to synchronize the flash. Depending on the design of the flash, the length of a full power flash will vary. In an SB800, the maximum flash lasts about 1/1050th sec.

The key point is that the flash, firing at 1/1050 sec is much faster than the curtains which move at 1/250th sec on the D200 (or 1/320th sec on the D300), so to synchronize the flash it must fire at some point after the first curtain has fully opened, but before the second curtain begins to close, so the sensor is completely exposed to the light from the flash coming through the lens. You may also see that you have a choice of when you fire the flash; either right after the shutter opens (front curtain sync) or right before the shutter closes (rear curtain sync). The timing of the flash has profound effects on the light trails when using very long shutter speeds. It will place the light trails either in front of the subject when using front curtain sync and behind the subject when using rear curtain sync. Rear curtain sync usually looks more natural for light trails.

Up to this point I have been only discussing the normal flash sync. However, Nikon has added a mode called 'Auto FP High Speed Sync'. You select this mode in the menu on the camera. In this mode, the flash duration is stretched so that it fires continuously for the full curtain travel time, ie, 1/250th sec on the D200. So, the the flash pulse that normally takes 1/1050th sec is now stretched across 1/250th sec. The official name for this operation uses the word 'Auto' in front of 'FP High Speed Sync', because in this mode the flash will sync normally and fire normally below the flash sync speed, but it switches automatically into High Speed Sync (stretched flash) when the shutter speed is set higher than Normal Flash Sync Speed.

This all sounds like a pancea until you find out that stretching the flash pulse and using higher shutter speeds causes the power of the flash to be reduced from what it normally is. In fact, the times where you would really like to be able to use a mode like this, like fast action sports in bright daylight, you usually can't, because the high speed flash sync mode is not powerful enough.

In fact, the power becomes less and less as you increase shutter speed, because the slit gets narrower and narrower. So, in this mode the flash is dependent on aperture and shutter speed, and if the shutter speed is increased to 1/8000th sec, the power gets so low that a subject would have to be less than four feet away.

However, when stopping action is not the goal, and a really high shutter speed is not required, then FP Sync can be very useful. In fact, Auto FP is excellent for shooting portraits in bright daylight. Then, you can use a shutter speed well above the normal sync of 1/250th coupled with a wide aperture to blur the background which greatly helps isolate the subject. I typically use camera A mode, ISO 100, FP Sync, f/ 2.8 and 1/1000th in bright shade, and I get a flash range of about 10-15 feet. If I am in really bright light, like on the beach, the shutter has to be around 1/1600th, and this reduces the range to about 8-10 feet which is still reasonable for portraits.

If a greater distance than 10 feet is needed in bright daylight, there is no choice except to use regular flash sync and accept the requisite smaller aperture. This happens frequently when shooting a party outdoors, where I typically shoot in camera S mode, regular flash sync, ISO 100, 1/250th, and f/7, and I get acceptable fill out to 20 to 30 feet or so.

118 comments:

Justis Photography said...

You are as always dead on with your observations however; I use this feature for dramatic bridal poses when I want to keep my sky super dark. This gives a "supernatural" effect and has been a HUGE hit with the bridal party. It only works for static subjects that like you said, are close but with posed shots this can give you some SUPER results. Let me know and I will email you a sample. I am a true sucker for dark blues in my skies and it drives me nutts when photographers sacrifice the sky for the subject. In other words, they blow out the sky in order to properly expose the subject and for me, the sky lends itself to the overall mood thus must be given its due. I love the blogs keep them coming you are teaching me more than you know.

Russ MacDonald said...

Lance (Justis Photography),

This is to summarize our email discussion about this.

The common way to darken the sky when shooting flash fill is to use camera Manual mode, set the shutter at max flash sync (1/320th on the D300), and stop down to f/11 or so. Then the small f/ stop will darken the background but your TTL flash will have plenty of power to fill the subject.

However, when you do this, the background will always be sharp because of the small f/ stop.

So, the only time High Speed Sync makes sense is when you want to soften the background with a wide aperture. Then you need to use wide open aperture and high shutter speed along with high speed sync. But this only works when you are very close to your subject, since the flash gets so weak in high speed sync mode.

Russ

Justis Photography said...

Thanks russ that is great info. I need to copy this and paste it in my reading files. You are the "Flash Daddy".

Lance

David Fein said...

Hi Russ,
I'm enjoying very much your clever descriptions and suggestions. I'm shooting with a SB-800 for some time. For some reason, you don't mention in your blog indications about the head angle of the flash or the usage of the diffusion dome/built-in diffuser. I know that changing the head angle or using diffusers affect the behavior of the transmitted information from the camera (D300) to the flash, i.e. focus distance, lens data ,etc. I would like to know if the flash exposure will remain precise in the cases explained above (angle and diffuser).
If you will have the time and patience I'll appreciate very much to get an explanation concerning the Commander Mode when using the SB-8oo as an external flash : which type of communication is transferred via the built in camera flash to the SB-800. How is it implemented, is the built-in camera flash light modulated? What happens when a translucent/reflective umbrella is used with the external flash?
Thanking you in advance,
David Fein

Russ MacDonald said...

David,

There are only some minor differences with the flash when you attach a diffuser or tilt the head. There is no change that I know of in the communication methodology between the flash and the camera.

When the diffuser is mounted on the SB800 the flash goes to wide angle mode and stays there, and will no longer zoom automatically as the focal length of the lens is changed. This makes sense, since when you attach the diffuser, the idea is to scatter the light as much as possible, and the wide angle position helps diffuse it even farther than the diffuser. If the flash were to zoom with the diffuser attached, it would fight against the diffuser, thus reducing the effectiveness of the diffuser.

When the head is tilted, the only change that happens is that the flash no longer shows a distance on its display. This makes sense since the only time that distance information would be accurate is when the head is pointed directly forward. When you tilt it, the system has no way to know how much reflection you will get from walls or ceiling.

When the flash is in Remote Mode there is no change in communication if the diffuser is attached or the head is tilted.

In fact, no matter how you modify the light from the flash, the flash metering in the camera simply monitors the central area of the image for reflected energy from the preflashes. Since the preflashes are highly calibrated, the flash metering can determine how high to set the flash power to make a proper exposure. Then, it sends this setting to the the remote flash(es) and they set themselves to this power.

If a diffuser happens to be attached to a remote flash, then the flash power will obviously have to be set higher to get a proper exposure, but that occurs automatically, because the preflashes also go through the diffuser and are made weaker by it, resulting in a weaker reflection to the flash metering system.

The same thing happens if you tilt the head up on the remote flash or you reflect the flash in an umbrella. Again, however you modify the light on the flash, it modifies the preflashes the same amount, so the system works to set the right power no matter what you do to it.

Now, there is a major change in the data that is sent back and forth between the camera and flash when switching between TTL and TTL-BL. This is because the distance information from the lens is used in TTL-BL, but it is not used in TTL (be sure to read my blog on this).

You can really mess up the flash metering system by attaching the flash to the hot shoe with a SC-29 cable and then placing the flash closer or farther from the subject than the camera and shooting in TTL-BL. Then, the flash metering system reads the distance from the subject from the lens, and it assumes the flash is at that same distance and adjusts it's power accordingly.

However, when the flash is in Remote Mode, the choice on the Commander is TTL, AA, or Manual, so TTL-BL is not an option when using remote flashes, so none of the distance information is ever used when the flashes are in Remote Mode.

Russ

David said...

Russ

I understand your logic for why you don't prefer to use Auto FP HSS. Now, assuming that if you do use it under the situation that you described, e.g., high ambient lighting where you want to open up the aperture to isolate the subject, then on the D300, what would be the relative advantages or disadvantages to set Auto FP in the Menu to 1/250s vs. 1/320s? Should I should set it at 1/320s in order to get the largest aperture possible? Thanks for a great series of tutorials on "flashography"

Russ MacDonald said...

David,
I do not own a D300, but from the D300 Manual, I can see that that the 1/250th is the highest sync speed when Auto FP HSS is turned off. Then, when you turn on Auto FP HSS, you have a choice of selecting 1/320th as the highest sync speed. I'm not sure why this is, but I think I would always select 1/320th if I had a choice.

The higher the sync speed before the flash goes into its 'pulse stretching mode' the better, because that means you get full power at a higher shutter speed. Once the shutter is increased above 1/320th, the flash power decreases greatly as it stretches across the entire period.

Yes, the higher the normal flash sync speed, the wider you can open the aperture in bright light and still do a normal full-power fill flash, which gives you more depth-of-field control. You are exactly correct.

If the ambient is so bright that you have to go above the normal flash sync speed to keep the aperture where you want it, you will control depth of field nicely, but you lose so much flash power that you often do not have enough to add the fill you need.

Russ

Anonymous said...

Russ love your blog. Between your site and Planet Neil I am really learning how to use my SB800 better.

I just read the section on Auto FP high synch and wish I had read it a couple of days earlier. I just shot an outdoor carribean festival and found that 1/250th of a sec was often not fast enough to freeze the wild movements of some of the dancers. Also I wasn't able to blur the background which was often busy and distracting. I understand that this would have lowered the output of the flash but since I was using the flash exposure compensation at -1 to -2 stop anyway would I have been able to use FP high speed by keeping my FEC at 0 or even +1 or +2?

Thanks for a very informative site
Ross

Russ MacDonald said...

Hi Ross,

Thanks for the nice feedback. I'm glad you find my work useful.

Auto FP High Speed Sync can work if the ambient light is fairly strong, and all you need is slight fill. However, if it is low ambient, the flash power probably will not be adequate to provide enough fill.

In any case, to have the best chance of Auto FP Sync working, choose the lowest shutter speed you can, that will stop the motion. In this case, 1/500th should have done it. This allows the power of the Auto FP Sync flash to be as strong as possible.

If you use a higher shutter speed than necessary, say 1/2000th, then you will reduce the flash power drastically.

Setting positive FEC will not increase the flash power once the flash is at max power, so I don't think that would help you. Normally when using FP Sync you need all the flash power you have, so I think it is best to just leave FEC at zero.

Hope that helps,

Russ

Anonymous said...

Russ, Thanks again for your prompt response. The lighting conditions at the parada were very strong, harsh sunlight so the auto fp high synch might have worked well. I will try this technique with the wedding I am shooting thisw weekend and see if I can get away with keeping my shuttters at around 1/500th. That should help with the motion blur problems though may still not allow me to use my f2.8 lenses wide open for a shallow DOF.

Once again, great site and thanks for the input. Look forward to reading more from you.

Ross

Fred said...

Thanks for your usual clear explanations, in the main text as well as in your responses to comments.

Readers interested in supplemental information might look at http://www.penmachine.com/2008/09/camera-works-shutters-flashes-and-sync and, to see a D3 shutter in slow motion, http://regex.info/blog/2008-09-04/925

A minor editorial point: Watching the D3 shutter in slow motion, the curtains, and slit, move from top to bottom, though your text says "bottom to top." It makes no difference photographically, and is irrelevant to your explanation, but, as a matter of accuracy, you might want to change it.

Russ MacDonald said...

Hi Fred,

I didn't really know which way the shutter traveled, but if it goes from top to bottom, fast moving wheels, like on race cars will lean backwards. I thought they leaned forwards on Nikon cameras.

Also, on that slow motion video, the shutter moves from top to bottom when you run the mouse from right to left, the shutter travel appears opposite to when you run the mouse from left to right.

I will do a little more research on this.

Thanks,

Russ

I will do a little more research.

Fred said...

Is the image on the sensor upside down and reversed left-right? If it is, then the final photo (which would take that into account) would show wheels leaning forward for a shutter slit that moves from top to bottom. See http://www.apug.org/forums/archive/index.php/t-25601.html

On the slow-motion "video" of the D3 shutter (actually an ingenious sequence of electronically-timed stills)the timing is shown at the bottom of the image, so you need to move your mouse from left to right. The sequence could have been reversed in assembling the video, but this seems unlikely, as the file names of the original stills encoded the sequence and timing.

Again, thanks for providing us with thoughtful, clear explanations throughout your website.

Russ MacDonald said...

Hi Fred,

Good link! It answered my questions.

I forgot that the image is upside down on the focal plane. That means that you (and the various people you referenced) are right.

The curtain definitely does move from top to bottom, which exposes the upside down image from bottom to top, which accounts for the rade car wheels leaning forward in images.

I will fix that in my blog. In fact, it will probably be fixed by the time you read this.

Thanks for your help with this!

Russ

Anonymous said...

Wow Russ, i've learned a lot from this post and the comments! The fact is that i just bought a D300, which is the first body i own that supports High Speed Sync Flash so i'm eager to try it out. Unfortunatly, it doesn't work for me? Strange, because i've set the flash to FP High Speed Sync, attached my SB800 in TTL, opened my aperture so i'm having a fast shutter speed of 1/1000th (just for testing) and guess what: i can only see flash light at the bottom of the picture (only 1/10th of the subject is illuminated, the rest of the model is shot without flash light). Do you have any idea what could be the cause?

many thanks,
Kris

email: kris.vandevijver@gmail.com

Russ MacDonald said...

Hi Kris,

I'm glad you like my blog!

I hope there is nothing wrong with your camera or flash. Let's see if we can get it to work.

Note: I own a D200 but not a D300. They are very similar but not identical, so I am using the D300 manual to help me with this.

Here are the steps:

1. With both the camera and flash OFF, mount the flash on the camera hot shoe.

2. Turn on both the flash and the camera (order doesn't matter).

3. Go to menu e1 on the camera and select 1/250 s (Auto FP) or 1/320th (Auto FP). Hit 'enter' to save it.

4. 'Wake up' the flash if it is in STBY by half-pressing the shutter button. Then, on the flash press the 'Mode' button several times to cycle through all the modes. You should see 'TTL FP' and maybe 'TTL BL FP' amoung the modes. If 'TTL BL FP' is not shown, you probably have your camera in 'Spot' metering mode. Switch it to 'Matrix' to allow BL. Set the flash in 'TTL FP BL' mode for fill flash testing.

5. On the camera select Manual mode and verify you can set the shutter at speeds above 1/250, all the way up to 1/8000.

Now, you should be able to select camera A mode and shoot in bright light with fill flash.

However, the higher the shutter speed you select, the less flash power you will have.

Hope that helps,

Russ

Anonymous said...

Russ,

With slower shutter speeds for normal shots do you use the rear curtain setting or just leave the flash firing normally?

Russ MacDonald said...

To Anonymous,

I usually shoot in camera Manual when it is dark, and I mostly use rear sync when I want to show light trails or ghosting that ends at the sharp image stopped by the flash. If you use normal sync for these shots the the sharp image will precede the ghost or light trails which doesn't look right.

The only other time I use rear sync is when I want to use A or P mode when it is dark, to let the camera choose a slow shutter speed. Again the reason is to get the ghosting or light trails on the right side of the sharp image.

Russ

Anonymous said...

Thanks for posting this information about the CLS system.
As for the problem of losing flash power using FP and only 1 SB800, would using the SB800 along with 2 SB600's all firing forward to be able to stop action.
Either wired or using the wireless triggering of the 600's.
I have been trying to shoot my son riding a go-kart where the lighting is on the dark side and 1/60 is way to slow but FP flash is to dark.

Russ MacDonald said...

To Anonymous,

Yes, you can use multiple flashes in Remote mode with all set to the same Group to increase the range of the Auto FP mode.

Of course, unlike regular flash sync mode, the maximum range in Auto FP Mode always decreases with shutter speed.

I ran a simple little test here in my studio. I have only one SB-600, so I tried it with an SB-800 both in Remote mode and with an SB-800 on-camera in Commander Mode.

Then, I put the camera in Auto FP Mode and set all flashes, including the Master to TTL with 0.0 compensation. At ISO 400, 1/4000 and f/3.5, the maximum range was about 10 feet.

So, if you can stay within 10 feet of your subject, you should be able to use Auto FP mode with two SB-600's, as long as you stay below 1/2000 shutter and f/3.5 or wider aperture.

Russ

Anonymous said...

Thanks Russ,

I use a D80 so I think the max shutter speed for flash is 1/200.
Is it possible to shoot faster?
Looks like I'll have to visit HD tomorrow for some aluminum to make myself a bracket to support the 2 SB600's left & right of the camera and have the 800 on the hotshoe.
Then my poor son is going to have to go go-carting again.

Russ MacDonald said...

To Anonymous,

1/200th is the highest regular flash sync speed on the D80.

Auto FP High Speed Sync allows you to shoot at any shutter speed you want. The problem is that as you increase your shutter speed in this mode, your flash range goes way down.

That's why I gave you the information in my last reply. I wanted you to see what range you could expect from the flash at 1/4000th sec. You may only need 1/1000th sec to capture your son on the go-cart, so your range might increase to 20 feet or so with three flashes firing at once.

Of course, if the ambient lighting is dim, why not use regular flash sync and make the flash primary on the subject? You'll get much more range; out to 65 feet or so with a single SB-800. I would shoot in Camera Manual mode, at ISO400, 1/200th sec, and f/'wide open'. Then, the flash will stop the action, and 1/200th shutter will eliminate ghosting (assuming the ambient is dim).

Russ

Anonymous said...

Thanks Russ,
That is what I will try.
1/200 sec.
I will also have both 600's with me so that if the one 800 can't provide enough light, then the 600's should help.
I didn't realize I could shoot faster then 1/200.
Your explanation's are the best I've read yet about photography.
Flash in particular.
Now I need to go practice what I have learned.

Russ MacDonald said...

To Anonymous,

Thanks for the compliment!

Just to be sure you understand. You are limited to 1/200th on the D80 in regular flash sync. You have to change Custom Menu 25 to 'ON' to change the sync to Auto FP Sync before you can set your shutter above 1/200th. See page 98 of the D80 User's Manual.

1/200th and slower, you will get full power flash. Above 1/200th and your flash gets vey weak as we have discussed.

I normally leave my camera (D200) in regular flash sync. I don't find the Auto FP to be that useful because it makes the flash so weak.

Russ

Jeremy said...

Russ,
Got to your blog from photo.net by Ben Haslam. Thank you for your time to share your experiences.

I've been using F3 and a "auto"-manual flash for so long and gotten good at it. I've been waiting for a long time to go DSLR due to fact that digital image quality lags behind film till now. Not as good but not as noticeable. With a giant leap at Nikon R&D starting with the D40, D300, D700 (personally think that these were test/finacial DSLRs for their R&D) that when I finally decided to venture into DSLR, I discovered to my dismay that my flash photography was all out of snyc (pun intended).

Started out with the D40x/SB800 and then D300. Waiting for a good full-frame. (D800 maybe). Could not get the correct fill without the harshness or over/under-exposing different parts of the scene.

But after reading your blog, it opened up possibilities. I'm mainly an action photographer, weddings and stage performances(no flash). Back to school for me:)

Just to say Thank you and appreciate very much for sharing your skill with all.

Russ MacDonald said...

Hi Jeremy,

I've been getting quite a few comments from folks at photo.net.

DSLR flash is definitely different from SLR flash, but I think it works just as well once you understand it completely.

Thanks for the nice feedback,

Russ

Jerry Bui said...

Russ,

Thanks for your blog. I have a D90/SB600 combo. Does the AUTO FP setting to ON automatically sync my flash to faster camera shutter speeds or do I, once the setting has been enabled, still have to manually go into the menu and ratchet the flash shutter speed higher to match my camera shutter speed? I was under the impression that the sync was performed automatically once the AUTO FP setting was turned ON.

Thanks!
Jerry

Russ MacDonald said...

Hi Jerry,

Once you set Auto FP to ON in the menus, your flash will automatically go into FP mode whenever the shutter speed exceeds the Sync Shutter speed.

The menu item called 'Flash Shutter Speed' is the LOWEST shutter speed that the camera will automatically select when using flash and you are using camera A or P modes. In camera S and M modes, you can set the shutter to any speed you want to when Auto FP is selected.

However, you have to remember that Auto FP mode reduces the power of the flash from normal sync, so it makes it useful only in very specific situations. Auto FP is very useful for allowing a wide aperture to blur the background when shooting fill flash in bright ambient conditions.

Also, Auto FP is not good for much of anything in low ambient conditions, so under those conditions, make sure that if you have Auto FP set to ON, not to exceed the flash sync speed with the shutter; ie, don't allow Auto FP to engage.

Russ

Anonymous said...

Russ-

Thanks so much for these explanations. You're doing the Lord's work. Or the Nikon user manual writer's work. Either way, it's excellent and this post in particular has cleared away tons of confusion.

One question, if I may: using the pop-up as a commander (though I suppose it's the same with an 800 or 900 as commander), you have the option of changing the EVs of the groups. My question is this: what is 0? As in, if you leave it at TTL EV 0, what is that value? And the subquestions are, is dialing my key to +1 and fill to 0 (for a 2:1) the same as dialing the the to 0 and fill to -1 or key to +.5 and fill to -.5.

Anyway, if you can answer this, that would be much appreciated. Regardless, thank you so much for these posts.

Russ MacDonald said...

To Anonymous,

Thanks for the kind feedback!

0 on the commander means that a 'standard' brightness will be set by the TTL flashes in that group. 'Standard' means a flash setting that the computer in the camera thinks will spread the pixels evenly across the histogram.

Setting your two-group studio is somewhat of an art as well as a science, because the 'standard' brightness is often not right, so you have to adjust compensation to fix it.

The settings you listed will all adjust the lighting power to be 2:1ratio, but the overall brightness will be shifted up or down from 'standard' depending on where you set the main.

A 2:1 ratio means the main is twice as bright as the fill.

Also, it is important to understand that when you set the power of your fill to one stop less than your main, you get a 2:1 power ratio, but the classic 'lighting ratio' is 3:1.

Let me explain:

Let's assume you set your lights with your main to the right and your fill to the left of your camera and pointed at your subject in the middle.

Now assume your main is set to 0 and your fill is set to -1.

This means that where the main strikes the subject by itself, the subject is lit to 0.

And where the fill strikes the subject by itself the brightness is one stop down (half the brightness).

Now, here is the meat of the discussion. Where the two lights strike the subject together, they add, so the brightness is equal to 2.

Now, the 'lighting ratio' is defined as the ratio between the brightest spot on the subject and the darkest spot. The brightest will be where the two lights add, and the darkest will be where the fill light alone hits the subject.

This makes the lighting ratio the ratio between where the two lights strike together and where only the fill light strikes.

Therefore, the setup that you described with a 2:1 power ratio will give you a 3:1 lighting ratio as long as there is some portion of the the subject where both lights hit.

Does that make sense?

Russ

Renato Rocha Miranda said...

Russ,

Please, is there a way to calculate the correct flash power output when in FP mode?
When I use shutter speeds lower than 1/250s, I think it´s pretty simple, I set tha Sb to Manual mode, set the aperture I´m using and dial down or up the output power untill it reaches the distance where my subject is.
After getting all this data, I change to 'remote" and feed the commander mode in my D200 with the numbers I´ve just found.
When I try to use higher shutter speeds (I live in Rio de Janeiro, there is a powerful and tropical sun above my head right now)things get way to much complicated. As a former enginner, I can´t stand using trial and error to find the correct power level...is there a formula or a way to use my own SB to give me the correct answer?
Every time I chance the shutter speed there is a new "experience" to be done...It´s some what frustrating and I realy need this function in my job. Can you help me?

best regards, thanks in advance and apologize me for any mistake, I´m not a english native speaker

Renato
www.flickr.com/photos/800asas

Russ MacDonald said...

Hi Renato,

No, there is no way to calculate flash power when using FP mode, because the flash power is different depending on the background.

The calculations you describe for standard TTL mode are also not correct when shooting outdoors, because they do not take into account the ambient light. You will very often overexpose if you use the calculations as you discuss.

All calculated flash values assume that the ambient light will not contribute to the image. When the ambient light does contribute, then you are right back to trial and error. You have to guess how much to reduce the flash.

The nice thing about TTL-BL is that it takes the guesswork out of shooting fill flash. You just have to set the desired flash compensation and shoot. The flash automatically adjusts its power based on both the reflected preflash power and the background.

Russ

Mike said...

Russ:

This blog is a Godsend. Thank you!

I'm a relatively inexperienced strobe photographer. I'm primarily into nature shots using available light. I'm now looking to get my first "off camera" strobe to use for head shots for my wife and a few of her friends who are amateur actors (local commercials, that sort of thing).

I'm considering a single SB800 with a reflective silver umbrella, and then either a second sb600 or a possibly a simple reflector (depending on results I get with the reflector).

Here's my concern:

If I use CLS and I'm shooting into a silver reflective umbrella, and I'm setting the shutter speed relatively high to reduce my depth of field (using an F 1.8 Nikor 85mm lens), won't the pics constantly be underexposed...or am I missing something? I would think shooting into the umbrella would increase the distance to the subject (at least more than the camera thinks), as well as disperse the light more than the camera thinks. Maybe CLS has ways of handling this that I'm unaware of. Or maybe I'm just not understanding how CLS works.

Thanks for any "light" you can shed on my question (pun intended). :)

Russ MacDonald said...

Hi Mike,

Thanks for the nice feedback!

CLS is really quite remarkable, and placing the flash in a reflective umbrella will have no effect on the brightness of the subject as long as you stay within the power capability of the flash. In fact, no matter what modifiers you place between the flash and the subject, the system will adjust and automatically compensate for any attenuation.

The key is that the power of the flash is determined by the computer in the camera after it measures the reflected light from the monitor preflash from the remote flash. The camera measures this preflash through the lens (TTL).

This means that if there are any modifiers between the flash and the subject the monitor preflash will be attenuated just like the main flash will, so the power of the main flash will be increased accordingly.

So, the distance from the flash to the subject will not affect the brightness of the subject, within the power limits of the flash. Obviously, if you move it too far away or put in too much attenuation, the flash will go to full power and that may not be enough for a normal exposure.

Therefore, it is very possible that you may be getting dark images because you are trying to exceed the flash's max power.

In my work using SB-600's and 800's, I have found that f/5.6 is about the smallest aperture I can use when my flashes are in umbrellas. If I try to use f/11, the flash will go to maximum power, but that will not be enough to light the subject properly.

Just to be sure you understand; the little electric eye on the flash is not used when shooting TTL. That electric eye is used only in A or AA modes, and if you try to use one of those modes in an umbrella, all your images will be dark, because the power will be based on the reflection from the umbrella and not the subject. This is why TTL is so nice! TTL automatically sets the power to make the subject the right brightness, no matter what modifiers you may use (within the power capabilities of the flash).

Hope that helps,

Russ

Mike said...

Exactly the info I needed. Thank you! I just pulled the trigger on an SB-600, stand, and umbrella. As I understand it, my D-80 will also slave a second SB-600 if both SB-600's are both set to the same channel. Correct?

Thanks again for the blog. I googled the snot out of "Nikon Auto FP", and this is the only decent info I could find.

Russ MacDonald said...

Hi Mike,

Yes, you can put as many flashes as you want to in a group, and the Commander will fire them all in the same mode and at the same power.

However, I don't see why you would want to do that. The D80 can control two groups, so you can make one of your 600's the main light and the other the fill.

Put them in TTL mode on the Commander, and then it's a simple matter to set various lighting ratios and make some really good portraits.

Russ

Paco Romero said...

Hello,
This blog is excellent! I tried to use Auto FP with the onboard flash as the Commander set to "--" because I didn't want it to participate in the exposure. The camera didn't allow me to go over 1/320, which is what I set the flash sync speed set to. I didn't try setting the onboard flash to TTL, as I really didn't want this. I want the onboard flash to simply command the remote SB800 to fire with a shutter speed of 1/1000, but I'm not sure I know how to do this. I tried M, A and S modes on my D700 but the shutter speed would not go above 1/320. Any CLS experts can help? I find the Auto FP description the SB800 and D700 totally insufficient!

Thanks

Russ MacDonald said...

Hi Paco,

I'm not sure exactly what you are asking. I have explained in the blog article what Auto FP Sync is and how it works.

The shutter speed cannot be set above the flash sync speed unless Auto FP mode is selected.

Are asking how to select Auto FP mode? If so, it is done in the camera menus. I don't own a D700, so you will have to read your manual to find out how to select it.

Regards,

Russ

Mike said...

Hi Russ!

Well The two SB-600's came in. I mounted them all up on the umbrellas, followed the instructions that I found in your blog in combination with the manual.

I also did the following:

1) Enabled Auto FP on my Nikon D-80.

2) Set Sb-600's both to wireless mode. Same channel on both flashes, group A for flash # 1, Group B for flash #2.

3) Set the popup flash on the D-80 to commander mode. I set the popup to -- because I didn't want it to go off.....I just wanted it to command the other two wireless SB-60 flashes.

Here's what I found:

1) In commander mode with the popup flash on my D-80 set to -- (off) the pop-up flash still went off. I expected this not to go off at all. I finally set it to manual and 1/128 power, but I'm wondering why the flash was going off when set to -- mode. If it is indeed supposed to fire in -- mode, do you know at what power it is firing at?

2) No matter how close I set the flashes to the subject, and no matter how high I set the power on the flashes, (even with the Aperature of my 85MM lense set to 1.8), the flash speed never excedded 1/200th of a second.

I triple checked, and had enabled auto FP. Of course I was sucessful at vastly over exposing the photo, but that was all (ie, as far as I could tell Auto FP was simply not working). I expected Auto FP to give me much higher speeds. Is this a limitation of trying to use the built in popup flash as a commander?

Thanks in advance for any guidance you can provide on my questions.

Mike said...

Ooops....that should read the SHUTTER speed never exceeded 1/200th of a second.

Mike said...

Russ, one more thing. The TTL metering really didn't seem to work unless I set the camera to Aperature priority (which hard set the shutter speed to 1/60th of a second.

With the camera set to manual and using TTL, shouldn't the built in metering system communicate with the flashes such that I can use the D-80's built in light meter?

Russ MacDonald said...

Hi Mike,

This is normal. The pop-up flash will always send out the preflash sequence to command the remotes. When you set for 'off' it just doesn't contribute to the image. but it always has to fire.

Russ

Bernhard said...

Hi Paco,

I'll try to summarize what I did to set my D700 and my SB800 to Auto FP Sync:
1) Set the cameras Flash sync speed (custom setting e1) "to 1/320s (Auto FP)"
2) Set the Flash cntrl for built-in flash to "Commander mode" with Built-in flash mode to "--" (as you did) and Group A to mode "TTL"
3) Set the D700 to Manual exposure mode and dial rear control wheel to faster shutter speeds (on my setting I have to dial to the right to get faster speeds). Make sure you do not hit the "wrong end" of the scale reading "x 320" (which comes right after bulb)
4) Set the SB800 to remote

Hope this helps,
Bernhard

Mike said...

Ugh! I figured out most of the issue. I wasn't manually firing the modeling light first. That seemed to get the communications going btween the D-80 and the wireless flashes, executing the Auto FP and getting the exposure right.

Trial and error......works wonders.

Still the question remains about disabling the commander flash. Will the flash on the popup still go off when I have it set to -- (off)?

Bernhard said...

Hi Mike,

How did you try to set your shutter speed?

I guess the "flash" you are seeing from pop-up flash is the way how AWL works: Nikon seems to use the flash light (at a very low power) to transport information to the remote speedlights.

Bernhard

Anonymous said...

Russ-

Thanks for your excellent work. I've learned a lot from this blog and wanted to contribute a bit. In regards to one of the earlier posts about max sync on the d300, I believe the way it works is that the "true" max sync for the d300 is 1/250th. The pop-up is able to do FP at 1/320th and then can't do it any more, so that is why the 1/320th option is there. This is, however, FP and there is reduction of power output.

This is also why some of your more recent posters were having trouble, I think. They weren't disabling the pop-up contribution and so the camera limits you to the pop-ups FP max of 1/320th.

When I first got my d300, I had this exact same problem and question of why I was being blocked from going over 1/320th. It was always that I either forgot to change to Commander mode or that I forgot to press OK after putting the pop-up on ---. Usually the latter.

Hope this helps some people and thanks again Russ.

Russ MacDonald said...

Hi Anonymous,

Thank you for that explanation, but I'm not quite sure I agree.

The pop-up is not powerful enough to be effective in FP Mode, so the designers didn't include that mode.

If you set the sync to 1/320th, the reason you can't go above 1/320th shutter with the pop-up deployed is because the pop-up cannot operate in FP Mode.

And it's exactly the same with 1/250th. If you set the Sync speed to 1/250th, then you can't go above 1/250th shutter with the pop-up deployed.

In 1/320th sync mode, with the shutter at 1/320th the pop-up fires in normal sync at full-power mode, but more of the tail is cut off than 1/250th, which lowers the power of the flash slightly.

Even in normal sync mode, at 1/250th shutter, the tail of the flash (pop-up or external) is cut off, reducing the power to less than it is with lower shutter speeds (see my blog on this: http://nikonclspracticalguide.blogspot.com/2009/03/15-when-is-full-power-flash-not-full.html ).

So, at 1/320th, more of the tail is cut off, resulting in less effective power. This is true for both the pop-up and external flashes.

Russ

Kris said...

The FP high speed sync flash is great, but only if your flash is on the camera's hotshoe. When you want to use off-camera flash to get a different kind of portrait and you'd like a wide open aperture (for the narrow dof), then what? My wireless readio triggers don't support the FP high sync speed.

Russ MacDonald said...

Hi Kris,

I'm not sure about your radio triggers, but with standard Nikon CLS Wireless (IR), Auto FP High Speed Sync works just fine with the flash off camera.

Russ

jimm said...

Hi Russ, I can't thank you enough for your time. I have read your whole blog twice and now can use and reconize the need for TTL or BL. I am trying to photography fast action motocross in sometimes bright conditions, the exposure from when they are in the sky to in a dirt turn can be 3 or more stops. I'm using a D2X and have a couple of 800's. My question is what are your recomendations? I need the shutter speed of 500+ to stop the action but just need enough fill to light them.
Thanks again

Russ MacDonald said...

Hi Jim,

Thanks for the nice feedback!

Well, I'm not a sports shooter, so stopping action is not my main forte, but I will tell you what I would do.

Shooting motocross with small flashes is going to be tough, mainly because you can't get close enough.

For two flashes, I would try mounting both of them on a bracket above the camera, one connected to the hot shoe with an SC28 cord (the Commander) and the other in Remote mode. Then, I would set the camera in Auto FP 1/250.

This should give you just enough to fill their face out to about 25 feet. But it may not be enough flash to fully light the riders.

Use the camera in manual exposure mode set to the minimum acceptable shutter speed, and the corresponding aperture to expose the sky correctly and see if the flash will be strong enough to light the riders. If the flash isn't strong enough, you'll have to open the aperture to increase the ambient exposure and let the sky blow out.

Or you can use regular flash sync, TTL-BL, rear sync, and slow the shutter to allow ghost trails. Those types of pictures can be very dramatic.

Hope that helps,

Russ

Anonymous said...

Thanks for great input, this blog post really helped me!

Russ MacDonald said...

To Anonymous,

You are most welcome!

Russ

Nick McDonald said...

First I would like to thank you for your hard work on your blog. Very informative, and definately useful.

I had read your blog post today about Auto FP High Speed Sync today and was wondering to myself exactly how much power you lose when you go into it. So I did a little testing this evening.

Nikon D90 - max normal sync speed is 1/200, (1) SB600 at full power triggered off camera via CLS. F stop and film speed were adjusted until the exposures matched the control photo.

Results:
Control Photo 1: 1/200 f/16 ISO200
Compare Photo A: 1/250 f/16 ISO1250
Compare Photo B: 1/250 f6.3 ISO200

Flash to subject distance was 17.75 feet.

That's 2.3 stops lost just by nudging into FP high speed sync (The other third goes to the higher shutter speed). On the plus side, once your into Auto FP High Speed Sync, the exposure difference by changing shutter speed any higher is what it should normally be. Ie, going from 1/250 to 1/500 results in a one stop lower exposure for the same amount of flash power.

As a side note, it took 2 more SB600's and an SB900 all at full power to expose the subject the same.

Anonymous said...

Are there absolutely no other type of lights that have the capabilities of an SB800 ie auto fp high speed sync?
Flashguns just don't have enough power and using a whole group of them is not really a practical solution.

Russ MacDonald said...

To Anonymous,

No, as far as I know, the Nikon flashes are the onlt ones that will do the FP mode on the Nikon system.

However, if you don't need a wide aperture, you can maximize the flash power by keeping the shutter less than the flash sync speed (1/250th on the D200). This will keep the flash in normal sync mode and increase its power by about a stop.

Russ

John Meyer said...

Great blog!

Anonymous said...

Hi Russ

I used to own a Nikon D50 which had a max flash sync speed of 1/500 sec yet the most semi-pro and pro cameras can manage is 1/250. Why is that ? Why can Nikon not give us faster sync speeds in semi-pro and pro cameras but can do so in cheaper cameras like the D50 without using Auto FP sync mode ?

Anonymous said...

Does anybody know the actual technical specs of an SB 800 going off in Auto FP high sync, such as duration, speed etc as compared to when it fires in normal mode?

Russ MacDonald said...

The reason the D50 (and the D70, by the way) had 1/500th as a maximum flash sync speed si because they use a partially electronic shutter. In fact, many people have covered a couple of flash contacts, and it will sync up to about 1/800th sec.

Unfortunately, there were lots of other problems with the electronic shutters at higher speeds, so they were abandoned after the D50 and D70.

Russ

Russ MacDonald said...

About FP sync.

There has been a lot of speculation about just exactly what the flash does when it goes into FP mode. Some people believe that the flash fires in a series of brief pulses, but I can find no proof of that. In my best judgement, I believe the flash fires at a lower power state for the entire shutter period.

There are many examples of strobes that fire in lower power states. For instance, strobes on towers go into a low power state with much longer flashes at night to avoid causing night blindness to pilots.

So, the net effect of the flash in FP mode is that it acts as though it simply stays on for the full shutter period at a much lower power. This means that the faster the shutter speed, when in FP mode, the less light from the flash will get to the sensor. This means that both shutter speed and aperture will affect the exposure when in FP mode, whil in normal sync, only aperture affects the exposure.

Russ

Michael said...

Hi Russ,

I've more or less stumbled on most of this accidentally. But seeing it concisely written is a huge help.

If I'm reading correctly-- the internal flash on an D700 can't be used, even only to trigger remotes if you want Auto FP High-Speed Sync mode. You must have an SB-800 or better triggering the off camera strobes. Correct?

But here's a daring question... I have a Profoto 2400W pack with 2 heads. That's a whole lotta juice burning a hole in my lighting kit that would LOVE to be used in high-speed sync situations. Is there a way to take advantage of that power on a D700 in high-speed sync?

Russ MacDonald said...

Hi Michael,

I don't own a D700, so I can't be 100% sure, but with my D200, the pop-up flash can be used as a Commander to fire the Remotes in FP mode as long as the Master flash is disabled. All you have to do is make sure Auto FP Mode is enabled.

No, there is no way to make studio strobes like the Profoto you mentioned to fire in FP mode. You are limited to the flash sync speed, 1/250th on my D200 and D3.

Hope that helps,

Russ

Anonymous said...

Hi Russ,

first let me tell you that I really enjoy your explainations and they help me a lot.
I use FP on my D80+SB600+85/1.4 a lot on portraits and so I´m interested in tips from you ;o)
You say that you use it ie on the beach. But do you use TTL-BL or normal TTL together with FP for portraits?
I´m interested in what to use if I shoot portraits on ie a wedding (in a church (means when there is ambient light, but nor so much))?
I shoot my 17-55/2.8 and maybe some with 85/1.4 and use the SB600 with them. Is it best to use TTL-BL or TTL (and FP) with that?

Many thanks in advance for all your work here - keep writing ;o)

Chris

Russ MacDonald said...

Hi Chris,

For beach shots, I use TTL-BL coupled with FP Sync for close-ups (headshots). This makes it possible to use camera A mode and select wide open apertures, like f/2.8, for that creamy out of focus background.

However, if I move back a little farther, say beyond about five feet, I can't use FP Sync anymore, because the flash is not powerful enough to fill against the bright beach sun. Then, I have to switch to regular flash sync (turn FP OFF)and switch to camera S mode with 1/250th shutter selected and live with the smaller apertures that will be required, because the background will not be so creamy anymore.

Whenever I am shooting indoors, I switch to regular TTL and camera Manual unless there is a large window in the background, in which case, I use TTL-BL and camera A mode, so the outdoors will be automatically exposed correctly.

When I am indoors, and there is lots of ambient, I still use camera Manual and regular TTL, but I speed up the shutter to darken the background a bit, which also reduces the chance of overexposure on the subject.

Indoors, it normally doesn't matter whether FP is ON or OFF. Even when FP is ON, FP mode will not be selected unless you (or the camera) selects a shutter speed above flash sync speed (1/250th on my cameras). I have never needed to select a shutter speed higher than flash sync speed when indoors.

Russ

Anonymous said...

Many thanks Russ, that helps me. I´m only a privat amateur (do not take money; shoot for friends and fun only). I wondered because in some blogs you mentioned that you shoot ie weddings only in P-mode as you have not the time for M-mode. In others (like above) you tell that you use M-Mode - so I got confused a bit.
I want to shoot events (a wedding of a friend soon in a church). I want to get best out of my fast lenses and best out of the pics and am a bit afraid of using M-mode (usualy use A and P-mode).
Usally I use my SB600 and bouce; when bouncing not possible I use my Omnibounce (diffusor) on it and direct to the object (dialed down a bit ~ 0.7 and more). but sometimes the pics are still blurred...

Many thanks again and keep this blog up - it is the best I found so far (and I´m reading a lot in the web since years)

Russ MacDonald said...

To Anonymous,

When you are shooting with your flash, the decision about using P mode or Manual mode should be based on how much ambient light there is and whether that ambient will be enough to affect the brightness of the subject.

Rule of thumb: If the ambient is bright, like outdoors during the day, use TTL-BL and Camera P mode. If the ambient is dim, like indoors under artificial light, use regular TTL and Camera Manual Mode.

The whole reason for using P mode in bright ambient is to allow the aperture to close down and control the exposure once the shutter reaches its maximum flash sync speed (Be sure to have Auto FP High Speed Sync turned OFF). You can also use S mode instead of P mode if you set the shutter at flash sync speed (1/250th on my cameras).

To shoot a wedding you have to always watch the light. When you are indoors in artificial light don't be afraid of using Camera Manual Mode and regular TTL. The ambient contribution to the subject will usually be very low if you set ISO 400, 1/80th, and f/3.5. Then, the flash metering system will handle the entire exposure. Try this at home inside your house at night to become comfortable with it.

Then, when you go outside of the church (in the daytime), the ambient will suddenly become bright, so switch to TTL-BL and use P mode.

Be sure to keep Auto FP High Speed Sync turned OFF until you fully understand how to use the flash without it. FP Sync can be very confusing if you don't know how it works (or doesn't work).

Russ

Itamar said...

Russ,

in the "beach situation" when you have a light background and want to use F2.8, what do you think about using a Neutral Density (ND) filter to make everything darker and use normal sync speeds ?

Should it work ?

Regards,
Itamar

Russ MacDonald said...

Hi Itamar,

It all depends on how powerful your flash is and how close you are to your subject.

The ND filter will definitely attenuate the ambient and allow you to shoot at flash sync speed.

The problem is that the ND filter will also attenuate the light from the flash by the same amount.

So, it will only work if the flash is powerful enough to overcome the attenuation from the ND filter. And on a beach you normally need an ND with about 5 stops of darkening to allow you to use f/2.8at 1/250th sec. With 5 stops of attenuation, I don't think a speedlight will have enough power to add much visible fill.

Therefore, I think Auto FP is a much better solution most of the time.

Russ

Itamar said...

Russ,

good point.
But instead of 2.8, if I can get 5.6 it would be better then 11... Who knows ?
I'd love to see your tests about this ND thing with the competence I can see in the others posts.

Thanks !
Itamar

Russ MacDonald said...

Hi again Itamar,

Yes, but why not just use Auto FP?

That will get you f/2.8.

Russ

Itamar said...

Russ,

in a post, answering to Chris in July, 22 you said:
"However, if I move back a little farther, say beyond about five feet, I can't use FP Sync anymore, because the flash is not powerful enough to fill against the bright beach sun. Then, I have to switch to regular flash sync (turn FP OFF)and switch to camera S mode with 1/250th shutter selected and live with the smaller apertures that will be required, because the background will not be so creamy anymore."

So I guessed ND filters could be a solution.

Russ MacDonald said...

Hi Itamar,

Yes, what I wrote is correct for the brightest situations, but if it's not super bright, FP Sync will work out to about 10 feet or so.

Also, you usually only have to add a very tiny bit of fill flash to lift shadows and be effective. So, even if you are beyond 5 or even 10 feet, you still might get enough fill using FP Sync.

In any case, if it is so bright that FP Sync won't work beyond 5 feet, then the ND filter required will be so large that that won't work either.

So, either way, you have to switch to regular sync and just accept the smaller aperture and the increase in depth of field that results.

You can always use 'Lens Blur' to blur the background in photoshop, and if done correctly, it looks very similar to using a wide aperture. It is pretty hard, however, to get the same look as nice lens bokeh in the specular highlights.

Russ

Itamar said...

Russ,

I see !
The latitude problem caused by the diference between the very bright background and the subject, will be kept when a ND filter is used.

Thanks for your patience and attention.

I'm learning everything I know about flash, here in your site.

Regards,
Itamar

ruke1 said...

Russ, thanks for your detailed explainations about Auto-FP. I've been shooting night football and indoor sports for 20 years, and have to say this feature on the Nikon cameras and speedlights is a giant step forward.
I'd like to add my two cents for any bloggers if that's OK:
My favorite setting for night football or soccer on a D3 is 1/500th @ f2.8. I always have the D3 set on Auto-FP of course, and set an onboard SB800 on TTL, usually at -1. I've noticed that when I release the shutter I can actually see the flash hitting the subject through the viewfinder, even though the mirror is up during the exposure. I've showed this to other experienced photogs and we were all dumbfounded by it. The only explaination is that the strobe either comes on before the shutters open, or it stays on after they are closed, or both. Anyway, I get great results with that setting but it does tax the flash due to the distance the light must travel. The recycle time between frames is slower so you've gotta stick with single frame bursts every few seconds even with five fresh batteries, but that makes you anticipate and wait for peak action, which makes you a better sports photographer.

Russ MacDonald said...

Hi Ruke1,

Thanks for posting your suggested settings for night football with a D3. I don't shoot sports, so that is one area I don't know much about.

I like the FP mode too, but I have never tried it for night sports. I suspect that you also increase the ISO significantly or you wouldn't get much flash range in FP mode. What ISO were you using?

Also, the reason you see the flash hitting the subject through the viewfinder is that you are actually seeing the monitor preflash pulses reflecting back before the shutter opens.

At longer distances, you usually only see them when there is a highly reflective surface, stop signs, or reflectors on bicycles - or football uniforms.

Russ

PK said...

Hi Russ. Truly outstanding work you're doing here. The D300 hundred manual seems confusing. Will the 320 sync speed option fire at full power? I don't really understand why this is different from any of the other HSS options as I'm presuming that the 250 ss is the fastest the D300 will go.

Thanks
PK

Russ MacDonald said...

Hi PK,

The camera will sync with the flash at 1/320th without going into FP mode.

However, what is not explained at all by Nikon is the reduction in flash power when selecting a shutter speed between 1/250th and 1/320th.

Well, I wrote another post that discusses to the problem: http://nikonclspracticalguide.blogspot.com/2009/03/15-when-is-full-power-flash-not-full.html (15. When is Full Power Flash Not Full Power?).

It turns out that the full power flash pulse has a significant tail that extends out to 8 ms or so. The main flash pules takes only 1/1000th sec (1ms), but that time period only includes about 90% of the maximum power.

When you use a shutter speed of less than 8ms (1/125th sec) you begin to chop off the tail of the flash pulse, and by the time you get between 1/250th (4ms) and 1/320th (3.1ms), you are chopping off a significant portion of the flash pulse. This is why the flash power decreases as you increase shutter speed from 1/250th to 1/320th. The reduction is not nearly as significant below 1/250th, so most of the time you can ignore the fact that it has decreased slightly in that range.

So, when shooting at 1/250th to 1/320th shutter, it's best to use regular sync if you can, since the main part of the flash pulse (the first 1 ms or 1/1000th sec) is what stops the action. However, both modes give you approximately the same illumination.

Hope that helps,

Russ

Anonymous said...

Your blog is great. Interesting, well written, with good explanations. Thank you for all the effort. I'm a hobbyist and it really educated me.

About flash power in FP mode.

Let's assume a D90 with sync speed of 1/200.

As I understand, the difference between shooting at 1/500 and 1/1000 is curtain slit width.

In both cases, the flash in FP mode needs to stay on for the *same* curtain slit travel time (1/200) - so basically it is similar to shooting in ambient light.

So my conclusion is:
If shooting in FP mode, a one stop increase of shutter speed (e.g. 1/500 to 1/1000) forces flash power to be doubled (or alternatively: flash range decreases by a ~1.4 factor).

Did I get it right?

Thanks
Samuel

Ulf said...

Hi!
Sorry but why don't yo use BL for your situation. Even in combination with FP?

In your example you use it because he background is that brigth and you want to dark them up by making the shutter a short time.
But it is fill light too isn't it?

2nd question. For Sport using.
By 1/640 to 1/1000 the flash light should have an effekt too isn't it? (of cousre not as strong as in not fp area.
But better than nothing.
So why don't use it in sport too?

Thank you so much.

Russ MacDonald said...

Hi again Ulf,

I guess I don't understand you question.

This post is about Auto FP Sync.

In this post I did not discuss the use of TTL-BL because that was not the topic. However, I would definitely use TTL-BL with FP sync when the ambient is bright.

Yes, whenever you use TTL-BL, you are adding 'fill' by definition. That's just the way TTL-BL works.

TTL-BL works for FP sync as well as regular sync.

"2nd question. For Sport using.
By 1/640 to 1/1000 the flash light should have an effekt too isn't it? (of cousre not as strong as in not fp area.
But better than nothing.
So why don't use it in sport too?"

You are right. It will add fill to sports pictures.

The point I was trying to make is that the action must be stopped by the shutter, not the flash, because when shooting sports, the ambient is high the shutter speed will be high, and the FP flash will only add a slight bit of fill.

The resulting sports image will be 'ambient primary'. The only time the flash stops action is when you are shooting 'flash primary'.

Please review my other blogs to understand the distinction between flash primary and ambient primary.

Russ

makanlagilagimakan said...

Russ,

Thank you for this very practical guide you wrote. I kept reading it many times before using it in practice.

I have a simple question. Can I use The SU800 as Commander of my SB600s (or SBR200s)? I asked because I am looking to purchase an R1C1 set.

Thank you very much.

Russ MacDonald said...

Hi makan,

Thanks for the nice feedback!

Yes, the SU-800 Commander will fire the SB600 and SB200 speedlights wirelessly.

The SU-800 will also control the SB-800 and SB-900 speedlights.

Russ

makanlagilagimakan said...

Russ,

Thank you again for the answer, and I hope you don't mind answering another one.

In the case of Auto FP (High Speed Sync), can I use the SU800 unit for commanding the SB600s and other CLS enabled speedlights? Or it can only be achieved by using SB800 or SB900 as Commander?

Thank you again Russ ...

Russ MacDonald said...

Hi again Makan (just guessing that's your name),

Well, the only time you would want to use FP sync is in bright light (like daylight) where you want to use a high shutter speed so you can blur the background.

Since I almost never use umbrellas outdoors, I have never had a chance to try this. However, I see no reason it wouldn't work.

As far as the SU-800, it is no different than the Commander in a flash, except it has an IR pass filter over the flash tube to block the visible light. So, anything that will work with the flash as Commander will also work with the SU-800 as Commander.

Russ

Jim said...

Thanks Russ for a great explanation. Funny how this is topic is just a bunch of confusing rules when you don't understand it, and a very logical situation once you do understand it. I guess your EE background provides you with the "understand the problem" approach to the world.
I am also a technical professional (computer science) with a passion for photography on the side. Now if I could just figure-out how to light my subjects I'd be able to put this all together. I've got a pretty good book on lighting portraits, but I need one for wildlife. Any suggestions?
Thanks, Jim

Russ MacDonald said...

Hi Jim,

Thanks for the nice feedback. Yes, my MSEE degree and 26 years with Texas Instruments definitely help me with all this. I also was in the camera/flash system design group for many years, which adds lots to my understanding of the flash.

Unfortunately, as a wedding photographer, I really don't know anything about lighting wildlife, and I know enough to tell you that it is a difficult task, so I can't offer any information to help you there.

Russ

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Anonymous said...

Hi Russ,
I read through every comment and didn't find the answer to my question, but I did learn a lot about my D90 and my SB-800 and High Speed Sync.
I want to use my D90 with two Calument Travelite 750 studio strobes.
I have a radio trigger, that syncs up to 1/250th. So I turned on auto FP and then hooked it up and it wouldn't trigger the stobes, so I wired the Travelite to the PC connection of the triggering device and it seemed to work, but I'm not sure - it only triggered one, and it was way to bright.
I was thinking that the flash duration was short so speeding up the shutter wouldn't help, but I didn't try it.
So how do I sync a studio strobe with a D90 - wired or not wired

Thanks,
Bruce

Russ MacDonald said...

Hi Bruce,

You can't use FP Sync mode to sync your D90 with studio strobes, because the only way to get into FP sync mode is to make the shutter speed higher than Flash Sync speed (1/200th), and then the camera won't sync with external strobes.

You should be able to connect your radio trigger to your camera hot shoe, with the camera at any shutter speed 1/200th or below, and it will send out the signal to your radio receivers. You can't use higher than 1/200th, because the D90 max Flash Sync speed is 1/200th.

Another way to sync a studio flash to the D90 is to use the pop-up (or your SWB-800) in Manual Flash mode and studio strobes using optical slave mode. In this configuration, the studio strobes will simply fire when the D90 pop-up flash fires.

There is no PC sync output on the D90, so you can't use a wired approach unless you buy an adaptor that mounts in the hot shoe with a PC Sync output. Those are reasonably priced and are available from all major photographic supply houses. I usually buy that sort of thing from B&H or Adorama.

Hope that helps,

Russ

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tom said...

Russ,

Today I attempted to shoot a family outdoors. They were in the shade. Light meter reading was 1/125 at f/5.6 on their faces. Bright buildings and blue sky in background. I set the camera to Manual and shutter speed to 1200 to darken the sky. Camera (D700) was set to FP mode. I had the SB-900 about 15 feet away and set to manual. From this distance the light meter gave me f/9.5 when measured from a pop. Yet, I could not get any light on the subjects at all. The background was perfect, but subjects remained dark. Why did the camera not pick up that f/9.5 light that was bouncing off the subjects?

Russ MacDonald said...

Hi Tom,

Your light meter reading on the faces was 1/125 @ f/5.6, but you had the camera set to 1/1200 @ f/9.5. Not considering the flash, this means the faces would be almost 4 stops darker than metered. That's a LOT of required fill.

Then, with the flash in Auto FP mode, the effective flash power is so low at 1/1200 and f/9.5 that it simply can't add enough light. In fact, the effective power of the FP mode is so low that at 15 feet, at 1/1200 the aperture needs to be about f/2.8 to add acceptable fill.

I'm not sure if there is any way to use a meter to measure the flash in FP mode, since the long flash pulse will mess up the meter reading. In FP mode it is ON for about 1/250th sec for all speeds above 1/250th, so you have to treat it like we used to treat fast flash bulbs.

In your SB-900 User's Guide, on page F20, is a table of Guide Numbers for FP mode. The table is for 1/500th sec, so the calculated f/ stop will need to be opened up by a little more than a stop for the 1/1200th shutter. I don't know what zoom setting you were on, so I will assume 50mm. The chart shows the guide number for 1/500th at 50mm to be about 60 feet, which indicates f/4 at 15 feet. Then, at 1/1200 shutter, that will be a little wider than f/2.8.

So, in your case, you had to open up the aperture to allow the flash to have enough power to fill. Of course that would brighten the sky.

Normally, if you want to darken the sky, you have to use the flash in regular sync, so it will have enough power to add the required fill. Of course, this means using a shutter speed of 1/250th and using the aperture to darken the sky. It will often take a really small aperture; maybe f/22 at ISO 100.

Hope that helps,

Russ

tom rains said...

Russ, after reading my email that I posted so late at night (must've been half asleep), I realized that I forgot to tell you that the flash was set on Manual and was not connected to the camera at all. I was using Pocketwizards to fire the flash. It was putting out a steady f/9.5 light (measured by my Sekonic), since it was set to manual at 1/2 power and 12-15 feet away from the subjects. It was due to the 9.5 light from the flash that I set the camera to 9.5, figuring that the 9.5 from the flash was being sent to the faces, just as it was the light meter.

With the flash and camera not connected, I am assuming that the FP mode will not work?? Does the FP mode only work when you have the flash physically attached to the hotshoe or via a sync cord?

Saturday am, I got my wife in the backyard to duplicate the setup and as you said, only at f/2.8 could I get any light on her face. The flash was set at 1/2 power, 10 ft away, on manual again, with pocketwizard. The flash again read f/9.5 light, at her face, and when the camera was set to 9.5 just would not give proper exposure. In my studio, my main is at 9.5 and I set my camera at f/10 and it gives me a perfect exposure. So why does the 9.5 from the SB-900 won't do the same?

Russ MacDonald said...

Hi Tom,

Tell, you what. Email me directly, at russ@russmacdonaldphotos.com, and let me address your questions that way. I am trying to keep this blog focused on Auto FP High Speed Sync.

Thanks,

Russ

Richard said...

Russ,

Thank you thank you thank you! I finally understand what Auto FP High Speed Sync is. I know this post is old but my quest is new and thankfully Google led me here. I do have a question that I'm not sure of from your base post and responses:

If I'm running M or S mode at slower than 1/250 (D300), does it matter whether I select 1/250 or 1/250(FP) in menu item e1? In other words, does it matter if Auto FP HSS is enabled if I'm not pushing the shutter speed into that range? In a completely non-scientific experiment, the histogram seems to shift ever-so-slightly when I flip back and forth between those settings but I'm not convinced that's anything more than my failure to have coffee before trying it.

I can hardly wait to dig into the rest of your blog. Thanks again for taking the time to write all of this down.

-r

Russ MacDonald said...

Hi Richard,

I'm glad you find my posts useful.

This post is really not old at all. I only post a couple each year, and people ask questions all the time.

About Auto FP: It is perfectly OK to leave the Auto FP turned on. It will only switch to FP mode if you set a shutter speed above flash sync speed.

I don't know why you were seeing any difference at all between FP on and FP off with the shutter below flash sync speed. It's supposed to be identical.

The reason FP isn't on by default is that if you don't fully understand FP mode, it will bite you. Any time your shutter goes above flash sync speed, the flash begins to act like a continuous light source that is ON only when the shutter is open. That means that the flash will no longer will stop action (action must be stopped by the shutter), and its intensity is directly affected by the shutter (the higher the shutter speed, the less maximum flash you have available).

Russ

Don Wessel said...

Russ,

Hadn't looked at your writings since in Nikonians and glad I found you again. Trying to get more comfortable with flash.

Got you bookmarked now. Good explanations, I'll check back here a lot.

Don

Russ MacDonald said...

Hi Don,

Glad you found me. I seem to remember you on Nikonians. You should come back. There are lots of really great discussions there. No flaming allowed and always constructive, or they're taken down. It's a great site for Nikon owners.

Russ

cho said...

I just found your blog through a search and found your tips very usefull.
Thank you.
Khuncho

Russ MacDonald said...

Hi Cho,

I'm glad you find my articles useful.

Thanks,

Russ

Phil said...

Hi Russ,

Great series of posts here! I'm very grateful for your help to fellow Nikon CLS users. I've recently discovered how useful and fun Auto FP HSS can be in the right situations.

I have a few questions that I hope you might be able to help me with. If I pick up another SB-800 and mount both of them off camera in a dual bracket setup, (both set to Remote and controlled from a D300) and only one IR sensor can see the beam (from built in pop-up or SU-800) could I wire the sync port out of the "seeing" SB-800 into the other "blind" SB-800 and get both to function in HSS correctly? I know SU-4 mode on the blind one would only give me a full pop. Could I daisy-chain more together or use inexpensive wired splitters?

Would all this be too taxing on the whole system or should I look for a different bracket so both IR sensors can see the IR beam? I'd like to make an inexpensive "light stick" for an assistant to move quickly with doing outdoor wedding/portrait work.

Thanks again Russ for your insights and sharing your knowledge.

Phil

Russ MacDonald said...

Hi Phil,

Thanks for the nice feedback!

No, I don't think what you describe will work. Both CLS Remotes must have direct line of sight to the Commander.

How about staggering them on the bracket?

Maybe you can rotate them say 45 degrees and then turn the bracket 45 degrees. That might open up the back one to communication with the Commander.

SU-4 on the blind one wouldn't work at all, because it will fire when the other Remote fires its monitor preflash, which occurs before the shutter opens.

Russ

Art Zaratsyán said...

Very good article

Anonymous said...

Hello again Russ!
I know it's rude to pop in only when you need something but if you ever need to brush up on your swimming skills let me know ;)

My question concerns the loss of power in HSS vs having an ND filter on. I know you previously said "The problem is that the ND filter will also attenuate the light from the flash by the same amount."
However, some claim that you will still get more light from strobe working @ 1/250 @ full power with ND x8, than 1/2000 at HSS. I guess the question is: is the light attenuation of a 3 full stops ND filter really equal to the loss of light without ND but @ 1/2000 HSS?

David.

Anonymous said...

BTW, I have a D700 and SB-900

Alan Clifford said...

Excellent explanation. My lack of understanding of auto FP on my D7000 was really bugging me.

Russ MacDonald said...

Hi Alan,

I'm glad you found my blog useful!

Russ

Anonymous said...

Two other considerations. If you use an ND filter outdoors you can get wide aperture and moderate shutter speed. Of course the flash is attenuated by the filter, however if you get to below FP speeds the DOF control for portraiture becomes available.

Of course one could always find some old FP foil filled flashbulbs. Those were fun!

Anonymous said...

Have just found this info.
THIS BLOG IS BRILLIANT.
Have now found out why I couldn't get a shutter speed above 1/320th.
I'm trying to photograph some Blue Tit birds nesting in a garden nest. I'm taking the images in the sunshine and keep getting highlights on the birds inner wings which I'm told is not the best for a good photograph. Once again many thanks for this info.

Russ MacDonald said...

Hi Anonymous (wish you would sign your name),

Thanks for the kind feedback on my blog articles.

In general, you should not use flash when shooting a subject that is in direct sunlight. Anywhere the sunlight hits the subject at the same place as the flash, it will overexpose and often 'blow out'.

If your flashes are close enough to the birds, you should use Auto FP High Speed Sync, so you could select a higher shutter speed. However, be aware that in FP Sync mode it is the shutter that stops the action and not the flash. The flash stays on for the full shutter cycle, about 1/250th of a second, when in FP Sync mode.

Also, in general, you should use TTL-BL instead of regular TTL when shooting in bright ambient conditions. TTL-BL tries to balance the power of the flash to make the subject brightness equal the ambient brightness. This means the flash will fire at a much lower level and possibly avoid some of the overexposure.

If you could get shade the birds with a scrim, and then use TTL-BL, you would have much better results.

Frank C. said...

Many thanks for your valuable reply Russ.

Steve said...

Russ:

Outstanding blog! You have the gift of taking complex concepts and expressing them into easy-to-understand language. Well done!

I am an avid amateur and a team photographer for my daughter's high school drill/dance team. I had a question about use of Auto FP. I have a D700 and the SB-900 using the 24-70 2.8. As you suggest, the standard starting point for normal sync (250 or below for the D700) is to stop down the flash by -1.3 to -1.7. At those times I need to exceed the shutter speed of 250, which then invokes the Auto FP, should I not stop down the flash conmensation while in Auto FP mode? Should my starting point be 0.0 compensation value due to weaker flash output?

Steve

Anonymous said...

Hi Russ,

Just a bit of history I found helpful. I hunted high and low for a definition of what the FP stands for in FP sync. Your statement that "FP stands for Focal Plane" sounds authorative but makes no sense to me. Then I found the Wikipedia article at en.wikipedia.org/wiki/Flash_synchronization which ties the term back to the old pre-electronic era where some flash devices were designed to produce uniform light output throughout their flash. They were described as "Flat Peak" and allowed photographers to push beyond the camera shutter's X-sync speed limit. The electronic flashes now emulate this behaviour with a burst of rapid pulses and commonly retain the FP moniker. To me, understanding the history helps me make better sense of the FP abbreviation. Also, using such a flash is not limited to Focal Plane shutters alone although the letters still fit.

Lissajous

Russ MacDonald said...

Lissa,

I worked in a camera design group for many years, and we always called it FP Sync.

You see, for the old leaf shutters, we didn't need to stretch the flash pulse, because the leaf shutter could work up to 1/500 sec and it sync'd perfectly with the normal 1/1000 flash pulse.

In fact, I never heard of stretching the flash pulse with any type of shutter except the FP shutter.

Russ

Russ MacDonald said...

Lissa,

Also, it is not correct that the FP Sync makes a series of pulses. That is something that was published years ago and continues to be misunderstood.

The FP flash pulse is a continuous pulse - not broken.

It is created by pulsing the circuitry internally, but the output waveshape is continuous.

Russ

Sasa Milovuk said...

congratulations for the wonderful articles. One question that lo flash when the high-speed operation, working as a strobe light. Thanks in advance