Hi James,
Or when using accessories that modify transmissivity of light, such
as filters in front of or behind the lens.
Or bounced flash. Or with flash modifiers like filters/omnibounce, etc.
Or where the flash-to-subject distance is not constant.
So, just like reflective ambient metering, you still need a grey card to
use E-TTL accurately. Or to help the photographer make an intelligent
guess. And if it still doesn't work, use the E-TTL system's automatic
flash exposure bracketing :-)
Of course when shooting digital, you could just check with a histogram
or eye-dropper tool immediately post exposure :-) Or digitally dodge/burn
the image. Or model the subject in 3D and add additional lighting !!!
Cheers
Julian Loke
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