
Originally Posted by
uyarchie
i use is evaluative on my flash and center-weighted average on my camera or spot. the reason for this is that I usually meter for ambient light first, then meter the flash exposure, which is controlled by the FEC and FEL buttons, that way I can fully take control of the ambient light, at the same time control my flash exposure through E-TTL II. it is especially beneficial if you are using master/slave function of 2 or 3 canon flashes or off-camera shoe cord since you lose FP-sync and rear-shutter curtain features of ETTL-II if you're using full manual on external triggers (these are really great features IMO). i still retain these setting even with on-camera flash.
canon did not publish the algorithms of ETTL-II, unlike TTL, A-TTL and ETTL... so i'd assume average is the same with "center-weighted average" metering on your camera, which calculates exposure mostly in the center of the frame to get the average exposure of the flash-lit subject, (especially helpful if background is too dark or too bright) while, evaluative uses the 35-metering zones across the frame to get an average exposure. actually i think it doesn't matter since the flash exposure is linked through the active focus point in your camera. it usually uses that distance info to get a good flash exposure through the pre-flash.
ETTL-II is by the way underexposed by -2/3 of a stop (this is it's flaw AFAIK)..i find it useful to use FEC +2/3 on all my flash setting even on fill-flash and work from there. 430EX which uses ETTL produces dead-on exposures IMO. hope this helps =) happy new year!