When I am using a Canon flash in the E-TTL mode, how can I tell which flash power the E-TTL magic chose?
For example, if I got a correct exposure in the E-TTL mode, can I easily choose the same flash power in the manual mode (without a lot of trial & error)?
Is this information available anywhere (camera, flash, EXIF data)? Does it depend on the Speedlite model?
In particular, I would like to easily find out how close to the limits of my flash I am shooting. If I noticed that E-TTL is using almost full power, then I might want to increase ISO to play safe and avoid underexposure if the conditions change a bit, and if I noticed that it is using very little flash power, I might want to decrease ISO.
Of course I know that there is a green "flash exposure confirmation" LED. This way I can check that I had enough power in my flash – but by how large a margin?