nikon - Why is there a preflash from a built-in flash when using remote flash only?

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?

Answer

So, actually, the preflash is a two-way dialog between the control flash (in this case, built in to the D7000) and the remote units. There's a reverse-engineered (and several years old, so possibly slightly out of date) explanation of the Nikon optical protocol by Alson van der Meulen. (The site is offline but archived.)

Basically, the control flash fires a minimal pulse, which actually triggers a reduced-power response from the remote flashes. The TTL system in the camera measures this response, and uses it to calculate the right power levels for each. (This may or may not include the control flash.)

Then, the controller flashes again with a reduced-power series of pulses which digitally encode instructions for the remotes, and in response every flash that's included in the exposure fires its main pulse.

All of this happens so quickly that it's impossible (or at least extremely difficult) to observe it unaided.

However, the last control flash actually happens with the shutter open, so even though it's at low power, if you have a wide aperture, or if a reflection of the camera appears in the scene, it can actually be seen. I assume this is a necessity in order to get the timing to work out.

(This works the same way for Pentax as well, although without separate control groups. I assume Canon and Olympus are basically the same but I don't know.)