Ah, ETTL, that's the kicker. I only have manual flashes, and I am envisioning an ambient situation where there's not going to be something throwing off the camera's metering.I'm surprised to see everyone saying they only use flash in M mode. While I am normally in M for flash work, I sometimes shoot flash in Av mode with a locked ISO so that the changes in ambient light will be picked up automatically by the shutter changes. Of course, I guess that is assuming manual flash; many people probably stick to E-TTL?
Using Av for flash and with a locked ISO will affect the one thing flash photographers need to control the most... shutter speed. If we can't control our shutter speed, we can't control how much ambient is mixed with our flash.
IE: Bounced flash mixing with ambient at a reception, event or mixed lighting situation.
Yes, that's my point, let the camera choose the ambient via the shutter speed. You can control the ratio of ambient to flash with exposure compensation, and control the flash power with aperture. That way, when the ambient changes, the camera compensates while keeping the same flash ratio/power/etc.
Yeah that's great but you'll still have wildly different exposures if the meter catches a bright light, and guess what? You have to dial in expo comp, then flash expo comp. then again when the light goes away... It's great if you like to fiddle a lot with that.
I find it easier to just set my settings in M mode, then simply control the flash exposure comp of the guests with ETTL. Then I have control of the ambient and the flash will be variable to changing light to get a good exposure on the guests. This works way better and I can concentrate on composition more than twiddling dials all night.