One of the situations I run into regularly is that I am fairly far from a subject who is in a dim ambient setting, and there is something closer to me that will be affecting the flash metering. This could be a plant, a wall, another person, a wedding cake, or anything that can reflect the pre-flash pulses back to the camera.
For example, let’s say your subject is standing inside a dim room and you want to shoot a picture of her from outside the room through a doorway. If you simply set TTL on the SB-800 flash and place the subject in the center of the frame and shoot, the reflections from the doorway will make the system think your goal is to shoot the doorway and you will get a great shot of a slightly overexposed doorframe and the subject will be pitch dark. The doorframe will be slightly overexposed, because it is not in the center of the frame, but it will still cause the flash to reduce power drastically leaving the subject dark.
The trick is to first aim the head of the flash forward and remove any diffusers that don't push the little switch on the flash head (so distance information can be used by the flash). Then switch the flash to TTL-BL, frame the shot and hit FV Lock.
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