Affecting flash power with AF points?
Keywords: nikon, speedlights, lighting, flash, focus, af
Lately there have been several comments made by experienced photographers stating that selecting different focus areas in the camera will change the place in the image that the flash meters from. I knew that this wasn't true based on my knowledge gained by designing flash and camera integrated circuits. I knew that the flash meters the entire frame based on center weighted brightness. However, I needed some proof, so I took the following series of images.
First, I arranged a white panel and a black panel so that they split the frame exactly in two. These images are unedited, directly out of my Nikon D3, low ambient conditions, f/5.6, 1/200th, ISO 400, regular TTL.
In image 1 (below), , the focus point was moved to the far left in the center of the white panel (the panel appears gray, because the flash system has averaged the center weighted frame and chosen a power that balances the entire image to Nikon’s meter 12% gray reflectance.
In image 2, I moved the focus spot all the way to the right side in the middle of the black panel. Notice that this image is exactly the same exposure as Image 1) showing that the position of the focus point is not used for the regular TTL metering system.
In the next few images, I investigate the effects of the focus system on TTL compared to TTL-BL.
In image 4, next, (still in TTL mode) I moved the focus point to the far right on the rope extending below the hats, and allowed the camera to focus there. You can see that moving the focus point did not affect the flash metering. The subject (although now out of focus) is exactly the same brightness as the previous image, as is the background.
In image 5 I switched to TTL-BL mode and placed the focus point back on the statue. You can see that the statue is no longer overexposed, as the balancing equations try to balance it with the darker background.
In image 6 I remained in TTL-BL mode and moved the focus point to the far right so the rope below the hats would be in focus. Immediately you see a change in the flash power. The statue is now overexposed and the background is much brighter. This is what has been causing even experienced photographers to think that the focus selection has an effect on flash power. However, the change is not being caused by moving the focus points. It is being caused by the change in focus distance. In this image the hats are about 15 feet behind the statue, and in TTL-BL mode, the distance is the prime factor that is used to set flash power. So, the farther distance caused the power to increase.
In image 7, I left the flash in TTL-BL mode, and switched off the auto focus and moved the focus point back to the statue. the important thing to notice is that the exposure of this image is identical to the previous image. This proves that moving the focus point does not affect flash power.
In image 8, I moved the focus point back to the hats. Again the exposure did not change.
These images prove that moving the focus point does not affect flash metering in either TTL or TTL-BL flash modes. The change in exposure that has been observed in TTL-BL mode when moving focus points is due entirely to the change in focus distance as reported by the D lens.
Editors’ note: Contrary to popular belief, Nikon meters (and Canon’s) are calibrated for 12% gray reflectance, according to ANSI standards (ISO 2720:1974). The correction needed if you are metering off a Kodak 18% gray card is ½ of an f-stop.
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Originally written on September 16, 2013
Last updated on January 24, 2021
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User on September 23, 2013
Russ, unless I overlooked it, you failed to mention the camera's exposure setting (i.e. Spot, Center Weighted, or balanced). You also didn't mention which lens you were using (i.e. AIS, AFD, AFS); depending on the lens, it may or may not provide the camera distance information to be used in the exposure calculations. If metering is set to Spot, "Regardless of which focusing area is chosen the highlighted focus area becomes the active spot for calculating exposure." This clearly suggests the focus points on modern DSLRs can in fact play a direct role in exposure determination. There are also many other things which could have contributed to the exposure calculations. Without fully understanding how your tests were conducted, I will take your results with a grain of salt.
john mcdonald (fineshot) on September 21, 2013
In image 7, the focus was turned to manual (as JRP says) and the focus point was changed. However, It is not a surprise to me that the statue is overexposed; as Alan said, the mistake is that Russ failed to re-focus along with the change in focus point (the focus is still on the hats). And as Hayo said, correct exposure is down to focus DISTANCE not focus point. Without the focus distance information provided by AF, the camera will have assumed that the subject is still at the (previously focussed) distance i.e. the hats! Correct exposure in TTL or even TTL-BL flash modes needs distance information from focus performed by AF - it does not work when in manual focus.
Egbert M. Reinhold (Ineluki) on September 20, 2013
Learned again. Thank you this article, Russ.
J. Ramon Palacios (jrp) on September 20, 2013
Alan, The AF was turned off. Only the focus point was moved.
User on September 20, 2013
I think that some of the confusion may be due to a differences between camera models. I tried a similar experiment with black, gray and white pieces of paper taped the wall, so there was no difference in distance. I compared a D80 to a D7000 and found that the exposure of the D80 was more sensitive to whether the focus point was in the black, white, or gray area. The difference between the two cameras was a 1/2 stop or less, but it was consistent.
User on September 19, 2013
I dont want to sound picky, but the picture no 4 seems sharp at the bookcase, not hats with rope... It could be tricky, because the photos here are very tiny. Still, I did believe that TTL was partly calculating the flash power from the distance which should be pin sharp, not point of focus you choose......
Hayo Baan (hrbaan) on September 18, 2013
Dear Russ, your images only show that (focus) DISTANCE is what determines flash output. This is something we already knew to be the case (and explains why the statue in some of your examples is over exposed). Given this fact, the focus point DOES actually matter.
Pleasant Lindsey III (LindseyP3) on September 18, 2013
Now you've given me reason to try TTL-BL. Thanks, good article.
Frederic Hore (voyageurfred) on September 18, 2013
Very surprised with your Back Lit results Ross. I had assumed that the metering system would look at what light was coming in from behind the subject, and adjust the frontal exposure to compensate. But there is nothing back light, so the meter just exposed as if there was not there. Interesting results! Many thanks for sharing your knowledge an test experience Russ. Cheers, Frederic in Montréal
john mcdonald (fineshot) on September 18, 2013
I have a couple of points with regard to your interesting study. First, in your opening paragraph, you say ”different focus areas ....will change the place...that the FLASH meters from”. When a Nikon Speedlight is attached to a DSLR in anything other than “M” (Manual) mode, it is not the flash but the camera that does the metering. Secondly, you missed a couple of vital points in your conclusions about exposure relating to flash. The Metering Mode choice and what the camera itself does to "compute" correct exposure both pre and post-capture (based on the image brightness range) are crutial factors to take into consideration. Throw into the mix the fact that your D3 will use it’s “Scene Recognition System” to interpret your subject, and if you have it engaged Auto ISO will kick-in, and it is all a recipe for double nay even triple guessing as to what kind of exposure will come out of the shot in any auto flash mode! I think it is important to state however, that no matter what inconsistencies may arise with Speedlight use, it is still best practice to select a focus point that corresponds to the part of the subject that is most important - wherever in the frame that might be. I would recommend experimenting with FV (Flash Value) lock for optimum results.
Ruedi Staehli (ruedi) on September 18, 2013
Thank you Russ, apart from being very interesting, this helped me understand (finally) the difference between TTL and TTL-BL modes! Best regards from Bern, Switzerland
Robert Kim Holwick (hillsidekim) on September 18, 2013
Thanks again for an excellent article. I knew that Nikon's meter was not set for 18% grey cards, but never knew how much. Now I know 1/2 stop difference. Thanks for the info. Does anyone make 12% grey cards?
Alan Martin (GeorgCantor) on September 18, 2013
An obvious slip on image 7: the statue is not in focus.
Alan Martin (GeorgCantor) on September 18, 2013
Please elaborate on "meter 12% gray reflectance". Thank you.
User on September 17, 2013
This has not been my experience- I have had the opposite results- When I have subjects in varying distances and select the closer subject in the frame I get an accurate exposure on the focal point selected subject but when switching focal point to the farthest subject, the closest subject is over exposed. I will do some testing on my own to verify my findings, and submit what I find. Thanks for the post Russ- Doug
Robert Appel (anymouse73) on September 17, 2013
Excellent tutorial, the information is logical and well presented. Thank you for taking the time to explain. (I also never knew about the 12% gray calibration.)
Victor Rakmil (VR8) on September 17, 2013
Thanks. very helpful.
Richard Luse (DaddySS) on September 17, 2013
Thanks Russ, very informative once again and very thorough. Much appreciated.