TTL with multiple flashes - How does it work?

[ZapoTeX]

When I have time to look for the right setting, I use manual. However, when I have a flash on camera and another one in my left hand and I take photos of people at parties or similar, I do like TTL.

OK, so, on the surface it is fairly easy: flash in my left hand TTL, no comp. Flash on camera TTL, -1.3 EV. I'm reasonably satisfied with the results: the off-camera flash gives a nice 3D look, while the relatively strong on-camera fill makes the lighting pleasantly even. And I have two little dots in the subjects' eyes as catchlights.

But I want to UNDERSTAND WHAT'S UNDERNEATH.

Here's how I understand Nikon's iTTL with one flash:
1) The speedlight shoots out a little pre-flash
2) Camera measures the light that is sent back from the subject through the lens and decides power of the actual flash
3) Shutter opens, the speedlight shoots out as much power as needed, as per calculation in step 2

What if there 2 or more speedlights? What I don't understand is:
1) Are there actually multiple pre-flashes? And separate calculations for each speedlights? Or just one pre-flash and then estimates? Do estimates account for different positions of the speedlights vs. the subjects?
2) What if one of the two speedlights is not powerful enough to deliver the power needed? For example: at ISO 200 and F/8, my Metz in my left hand can very well light up the scene. However, if the on camera flash is simply the pop-up, it might not be able to deliver even at -1.3 EV. What happens then? Are both flashes proportionally powered down so that the -1.3 EV ratio is respected? Or is the off-camera speedlight fired at 0 EV and the pop-up fired at, for example, -2 or -3 EV?

It drives me nuts that I don't know how it works, even if what I'm getting is acceptable. I googled for it, but it does not seem to be a common topic.

Ciao & thanks a lot!

 

[KmH]

Do some reading - www.strobist.com

www.planetneil.com

The Nikon Creative Lighting System: Using the SB-600, SB-700, SB-800, SB-900, SB-910, and R1C1 Flashes

The Hot Shoe Diaries: Big Light from Small Flashes

The Moment It Clicks: Photography Secrets from One of the World's Top Shooters

Sketching Light: An Illustrated Tour of the Possibilities of Flash (Voices That Matter)

 

[ZapoTeX]

Hi Keith! Thanks a lot!

On the free sources you recommend there are detailed explanations of how to use them, but not much about what happens inside. I guess I'll need to buy some of the books!

Thanks a lot for your recommendations!

 

[KmH]

The details of what happens inside them is proprietary camera maker information. The information is not published.

Third party gear makers have to 'reverse' engineer those details because the camera makers won't give it to them.

But, by acquiring a sound fundamental understanding of how to use proprietary TTL, one can then use logic to make some likely close to the mark assumptions about what does go on inside.

For example (and just speculation): Your D90 has a 420 segment RGB metering chip. Logically then, your D90 in iTTL mode could detect 420 a maximum of 420 levels of light. However I bet it is far fewer than that and closer to the 11 focus point locations you can place the spot used in spot metering mode. (I doubt spot metering mode is used for iTTL)

A D7000 has 39 auto focus points and can likely detect more iTTL light levels than your D90 can.

 

[ZapoTeX]

Thanks a lot for the additional answer!

I guess I'll find my answer in either the books or some experimentation with my camera. Or, even better, both and compare findings.

Thanks again!