Flash Exposure Lock (FEL) for repeatable TTL exposures
This is an example of using FEL to set the exposure for a headshot using ETTL and then activating FEL to keep the exposure constant from shot-to-shot. This is camera controlled FEL so it should work regardless of the ETTL speedlites being used. That will require confirmation through testing with others when they become available.
Photographers often give ETTL flash a hard time because it can deliver exposure variations from shot-to-shot, even if the subject and background are the same. It does happen. Especially if the TTL flash exposure reading is using average or evaluative metering.
The way around it with my Sony A7RII is to set the button inside the AF/MR AEL selector to FEL toggle. That permits me to press the button with the switch in the AEL position to fire the speedlites to take an exposure reading. That reading is then used for as many frames as I choose until I press the button again to toggle FEL off.
To confirm that the ETTL metered and locked exposure was repeating, I changed the Flash Exposure Compensation (FEC) settings for the A group and B group between shots. No change in exposure from either group. The lighting setup was Gr A in an umbrella framed softbox camera left as main and Gr B was in a translucent background box behind Manny. Gr B was set to FEC +1 when the metering occurred.
For all the time I was shooting with Canon cameras I never tried this approach. I don’t remember how FEL works and if it can be set so that it remains on shot-to-shot. Having never shot with Nikon digital gear I have no idea how it works with Nikon cameras either.
I did some testing in High Speed Sync (HSS) mode and it works the same way. The only caveat is that since the speedlite is operating as a constant light source the shutter speed with impact flash exposure as well as ambient. This can however be a benefit since setting the flash exposure via FEL, the shutter can be used to fine tune the exposure overall once the desired balance is found for the shot.
As one would expect crossing the threshold from FP-sync speeds to X-sync speeds kills the FEL exposure setting.
One question that came to mind after reading a post on one of the fora was how long the FEL stays active on the Sony 7ARII. I did a test that included 35 frames shot over about 4 minutes. During the shooting I changed the camera position from vertical to horizontal, changed zoom, changed the FEC for both Groups from the settings used for FEL to full minus. All the images have constant exposures. There is no reason to expect that the FEL would have automatically switched off until the camera went to sleep from inactivity or was turned off.
Give it a try and have fun!