Understanding ETTL Logic

I've been reading around and I'm trying to understand the Ettl system and ratios.

From what I've gathered, group "A" is the master channel - the power is derived for this group, then B is set accordingly regardless of any other parameters (like distance)?

So if I have a subject and place a single group A flash at 2 feet 45 degrees camera left, and group B single flash at 2 feet 45 degrees camera right, and set my ratio 2:1 (B is half the power of A) and take a photo. And I leave all settings the same but move flash B to a distance of 4 feet, and take a shot, the image will look different correct? The resulting image will look as if I had set the ratio to 8:1 correct?

In addition, do the camera metering modes affect the ETTL exposure levels? For instance, if I have a black background and a relatively small white subject and I shoot with a single flash, at both Spot, and evaluative the resulting images will be different correct?

Finally - with the Pottix system, should the CAMERA flash setting be set to average or evaluative?

I do not know if calling A the master group is correct or not. Any flash that is in the hot shoe as a master will be in group A. Just before the actual exposure the master flash commands a preflash from group A and then one from group B. Both exposures are measured and the correct one is guessed at by the camera. There are lots of variables that could change the final outcome. There is no magic bullet in eTTL. If in your example the flash in group b was already firing at or close to full power, it cannot make more power to make up for moving it back to twice the distance. Moving a flash back twice the distance requires 4 times the power for the same exposure.

Buchinger wrote in post #16023648
So if I have a subject and place a single group A flash at 2 feet 45 degrees camera left, and group B single flash at 2 feet 45 degrees camera right, and set my ratio 2:1 (B is half the power of A) and take a photo. And I leave all settings the same but move flash B to a distance of 4 feet, and take a shot, the image will look different correct? The resulting image will look as if I had set the ratio to 8:1 correct?

My take is that individual flash-to-subject distances are not factored in when using ratios. The group A and group B units fire their preflashes separately and the camera's metering system evaluates each flash's contribution to the overall exposure and sets each flash's power accordingly. So in theory you could change one flash's distance from the subject and still maintain the same ratio set in the camera's menu or on the master flash.

I experimented with both E-TTL and manual off-camera flash when I purchased a second flash about a year and a half ago. I noticed ratios were maintained even though one flash was more powerful than the other. Even manual power settings, using Canon's optical system, seemed to be equalized between flashes of differing power.

Of note are posts 1 and 3. Most of the responses indicated the posters didn't understand my question:

https://photography-on-the.net …/showthread.php​?t=1144295

ETTL ratios are independent of speedlite - subject distance as long as there is sufficient power to provide the need light. As mentioned above, the camera is metering the light from each source and then setting the power in each source to provide the specified ratio. Thus a 430 can be the Group A light and a 580 can be the Group B light and they will still deliver the request ratio.

Group A is always the speedlite on the camera (master) and any additional lights also designated Group A. Group B are slave lights, as are Group C. Group C can not participate in the ratio but its power can be controlled independently via Flash Exposure Compensation at the Group level.

All of this changes when using 600Ex-RTs in the radio controlled Gr mode.

Buchinger wrote in post #16023648
Finally - with the Pottix system, should the CAMERA flash setting be set to average or evaluative?

There is no right answer to this one. Evaluative tends to work better with subjects where there is ambient to balance, average for light subjects in dark rooms where your flash is the primary light source. Your choice has little to do with the triggering system, but the circumstances of your scene.

Gonzo - to be clear (perhaps you know this) - I was talking about the in camera external speedlite menu, not the regular metering mode of the camera. I've read that choosing the wrong one in the camera menu will cause the triggers to go haywire.

Buchinger wrote in post #16024288
Gonzo - to be clear (perhaps you know this) - I was talking about the in camera external speedlite menu, not the regular metering mode of the camera. I've read that choosing the wrong one in the camera menu will cause the triggers to go haywire.

I hadnt heard that it would affect the triggers. Although I have read more on the YN triggers than the phottix. But yes, I was referring to the optimal settings on the speedlite menu.

Buchinger wrote in post #16023648
I've been reading around and I'm trying to understand the Ettl system and ratios.

From what I've gathered, group "A" is the master channel - the power is derived for this group, then B is set accordingly regardless of any other parameters (like distance)?

So if I have a subject and place a single group A flash at 2 feet 45 degrees camera left, and group B single flash at 2 feet 45 degrees camera right, and set my ratio 2:1 (B is half the power of A) and take a photo. And I leave all settings the same but move flash B to a distance of 4 feet, and take a shot, the image will look different correct? The resulting image will look as if I had set the ratio to 8:1 correct?

In addition, do the camera metering modes affect the ETTL exposure levels? For instance, if I have a black background and a relatively small white subject and I shoot with a single flash, at both Spot, and evaluative the resulting images will be different correct?

Finally - with the Pottix system, should the CAMERA flash setting be set to average or evaluative?

What flash you are using has nothing to do with the flash metering mode one should use. It's the camera that determines the exposure based on the flash metering mode selected, not the flash.

When Average mode is selected, the flash lighting from the whole frame is averaged out to arrive at an exposure. For a relatively small subject, for example, a dark or a bright background can throw the exposure off for the subject so Evaluative mode might work better in this case.

But, as was pointed out earlier, there's really no right or wrong answer. It's best to experiment with each mode for the type of environment you use the flash and see which one works out better for you.

I agree that its a personal decision.
Canon tech reps I've talked to all suggest Evaluative.

That's what I use and have very good luck with ETTL.

But, like everything else related to lighting, experiment, test and practice will help you determine what works best for you.