580EX and White in P Mode?

[d'homme]

Has anybody had any experience with the 580EX and 20D properly exposing people in white clothing?

I've been reading that the 580EX is by default set to underexpose. Is that true?

[tim]

You need to understand how cameras do exposure, it's the same whether or not you use the flash. I don't have time to explain it, but basically for scenes with a lot of white you'll need to use EC (or FEC with a flash) of +1.5 or so, for scenes with a lot of black you'll need -1.5 or so. Hopefully someone will have time to explain why.

[rent]

i'm bored at work... so let me try to explain. :p

the metering system in our cameras always assumes the scene is 18% grey. so if a scene is predominately white, the meter still thinks it's grey, therefore the metering result actually underexposes the white scene. so you would need to dial in +1 or +2 stops to compensate (this is known as exposure comenspation (EC)).

the reverse is true for a scene that is predominately black -- you compensate with -1 or -2 stops of EC.

and without going into the details of flash metering (check out the sticky in the flash forum), the basic idea is the same, only here you would dial in flash exposure compensation (FEC) on your flash unit (or on your camera body) -- + for white, - for black.

-alex