When using Nikon D7000 with an off-camera SB600 (which cannot be used as commander) in TTL mode, the built-in flash must be opened to make it work (to serve as a commander), even if this built-in flash is set to off.
This built-in flash does not trigger when taking a photo, but does preflash. Why?
Searching for an explanation, I found that this is required to measure something. But what?
- A shutter speed in aperture mode? In this case, the speed will be completely wrong, since during preflash, the scene will be lighter than when taking photo, since in this last case, the built-in flash will not trigger.
- Built-in flash intensity? But why do we need it, since the built-in flash will not be used while taking photo?
- Remote flash? Again, the measure will be wrong, for the same reason as in the first point.
What I'm missing?