How does the wireless flash protocol control exposure corrections?

Question

The Panasonic Lumix series uses the wireless flash protocol developed by Olympus. The protocol features multiple groups of flash lights than can be controlled individually.

I have a question regarding exposure compensation:

• When using the built-in flash things seem obvious: If I want to flash to be at "-1", the camera probably makes a short "test flash" to estimate the reflected energy,compares it to the ambient light, and then computes the energy for the final flash (I guess).
• Now when I add one external wireless flash, maybe configured with "-2/3", how does the camera know how much energy that flash has to send to contribute "-2/3" to the overall exposure? My only guess is that it also has to fire a test flash at a different time than the built-in flash. And obviously to avoid over-exposure, the built-in flash also has to reduce its energy due to the contribution of the additional flash.
• Eventually when I add a second wireless flash group (for simplicity say just one flash) at "1/3", how does it all work together?
• When making the eventual shot, do all those flashes fire simultaneously, or do they fire in rapid sequence while the shutter is open?

Clarifications

I think I have to add some more details or facts:

• First I'm talking about the flash-based "wireless" protocol developed by Olympus (AFAIK) and used by Panasonic. I couldn't really find any details, but my guess it's a kind of serial protocol where bits are indicated by short flashes.

• Second I can actually choose between TTL, Auto, Manual, and OFF for each "channel", so I don't believe that the flash is simply set to some fixed reduced ratio.

Test Shots

I made a series of test shots using fixed aperture f=11 and fixed speed 1/60s. To avoid over-exposure in case all the flashes would really add, I set each group to TTL and "-1":

In addition to that I measured the light with a X-Rite ColorMunki. Due to holding the Munki in my hand, there was a slight variation in distance and angle, resulting in inexact readings, most likely.

The test setup was like this:

• Built-in flashlight was in the middle (obviously). Set to "TTL -1".
• Group A flashlight was a Metz 58 AF-2 positioned right of the camera, pointing towards the subject and using wide-angle diffuser. As the distance was short, I put some gaze in front of the reflector to avoid over-exposure. The secondary reflector was disabled. Set to "TTL -1", too.
• Group B flashlight was a Metz 52 AF-1 positioned left of the camera, pointing towards the subject and using wide-angle diffuser. Set to "TTL -1".

The only post-processing I did was to crop the images a bit, rescale them and reduce the JPEG quality (basically just to reduce their size). I also added the flash measurements (See "EV" or "XYZ").

In the background you see a part of the gray card of a DataColor SpyderCheckr24.

First shot was with built-in only:

Second shot was with built-in and group B (left):

Third shot was with built-in, group A (right) and group B (left).

I had the impression that the flashlight fire sequentially.

Finally I made a shot with just the ambient light (in case it would play a role):

Answer 1

The metering flashes are popped off one at a time before the shutter is activated for the exposure. The sequence is so rapid that it looks to our eyes like a singular pop, but it isn't. I don't know specifically about Olympus/Panasonic, but in general most TTL systems use a fairly low power pulse to measure the reflectivity of things in the scene. This preserves as much as is possible of the energy loaded into the flash's capacitors for the actual exposure.

When using off-camera flashes with systems that use optical control (instead of radio), there is a second pre-flash (it's actually an encoded pulse) that tells each group what power to use and gives a timing instruction letting them know how long to delay until the flash is fired for the exposure. The system then times the shutter release to be in sync with when the flashes have been told to fire.

Once the camera has metered each pre-flash, calculated the power needed from each group, and transmitted instructions to each group the shutter (whether mechanical or electronic) opens and all flashes pop at the same time.