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How-to's Accessories Reviews

TTL and TTL-BL - Distance and Subject Positioning

Russ MacDonald (Arkayem)

Keywords: nikon, speedlights, lighting, flash, sb_800

After several discussions on the Nikonians Speedlights & Lighting Forum concerning how TTL and TTL-BL work with respect to off-center subjects and distance, I thought it was best to do a practical study to really find out for sure. Until then, all my comments on this topic were based on what I've seen in my wedding photography. Now I can say for sure exactly what is taking place.

This is a practical study more 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 and simplify its practice, 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
SB-800 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.



In Image 1, only available light was used, 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 is also for reference. It also shows how the Auto WB gets the color right when flash is used. The histogram shows a similar pixel distribution as in Image 1.



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Mary Ellis (Mael) on August 24, 2013

Although I've had my Nikon for several years, I just joined Nikonians a few days ago, and signed in for the first time tonight. This was the first thing I saw as I came on board, and I must say, it was a great welcome! Until now, I don't think I've ever fully understood the difference between TTL and TTL-BL. Thanks for doing this study and presenting it.

User on August 19, 2013

Very informative. The visuals of the subject vs. the histogram are great for comprehension and retention.

Richard Luse (DaddySS) on August 11, 2013

Ribbon awarded for his generous support to the Fundraising Campaign 2014 Ribbon awarded for his generous support to the Fundraising Campaign 2015 Ribbon awarded for  his generous support to the Fundraising Campaign 2017 Ribbon awarded for his generous contribution to the 2019 Fundraising campaign

Very informative and interesting, thanks again and well done Russ!

David Goldstein (dagoldst) on August 10, 2013

Russ, Another good article. All the information about the Nikon flash system you have documented has most definitely improved my images. Thanks for all the hard work.

Victor Rakmil (VR8) on August 8, 2013

Very informative. Thank you.

Jerry Klumpp (jwk1940) on August 7, 2013

I have to agree with the Randal, in that image #11 does not match #8 as far as brightness, so judging from what I see, focus distance does appear to make a difference. Great that you took the time to do this and pass it along to everyone. Thanks

Randal Lathrop (perlnut) on August 7, 2013

Looking at slide 11 where you say that distance has no effect on TTL flash mode, the picture doesn't bear that out. It is definitely brighter than slide 8, and looks identical to slide 10. Other than that, great explanation of distance interacting with TTL/TTL-BL, and understanding what happens with small (relatively) bright objects with dark backgrounds. Helps to understand the practical use of the flash modes and understand what takes place "under the hood."


Very good informative flash test.

User on August 7, 2013

Like the flash test and the article. Thanks for sharing this information.

Patrick Galligan (pgalligan) on August 6, 2013

Great article by the way :)

Patrick Galligan (pgalligan) on August 6, 2013

Where were you focussing? On the blue towel or on the white target?