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?
Answer
I don't think that data is available.
However, I've found that you get a feel for good starting point just by using your gear. Once you get to know your gear you can look at a situation and say to yourself, "OK, I'm going to have my flash about 6' away, ISO 400 and f/5.6... I'll start with the flash at 1/8th power and adjust from there".
That kind of approximation is likely more useful than trying to use E-TTL to meter it for you - especially since it can be tricky to know that the TTL is metering the right area of your composition. If you really need an exact reading you'd be better off picking up a proper light meter that can fire your strobes.
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