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
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.
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