Found this great link which basically explains everyting there is to know about flash fotography with canon EOS cameras. Page 2 is especially recommended. Heaps better than canon's manuals.

Two things that comes to my mind that arent't 100% clear and perhaps some of you flash experts might tell me:

1 In ETTL II can you still autofocus on your subject and then re-compose to take a shot without effecting the flash exposure?? Flash metering systems are biased towards the focusing point in TTL and ETTL, wonder if any of you have done some expertiments there??

2 Also for my non-flash photography i use the centre focus point and centre weighted average for exposure. In flash photography the camera's exposure metering meters for the background whereas the flash does the foreground/subject. Will I still be able to use that or am i best to switch to evaluative (on the camera, as the flash metering is evaluative for ETTL2) and automatic focus point selection?

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1. Yes, as the Flash gets its exposure after you hit the shutter button.
It sends out a pre-flash to check exposure.

2. I wouldnt worry about metering too much, ideally if the lighting stays the same you shouldn't need to adjust your settings much anyway. Set your camera to expose for ambient and then the flash will expose your subjects.

1. What I meant was, imagine for instance you’re trying to take a photo of a person at the sunset. You put the person in the middle of your frame to focus, your background is the sky which is still bright enough. You press the FEL button. You then recompose the frame so the person is on the side and your background is now a dark tree. You then take a shot.

The person will be exposed correctly since you’ve locked the FEL exposure. The flash exposure works for the foreground whereas the camera exposure metering works for the background. Now:

A. will the background be exposed incorrectly since you locked the background exposure on the sky
B. OR because you’ve used FEL it only locks the exposure of the foreground (exposed by the flash) and the background gets metered separately upon pressing the Go button?

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Member of the 18-35 club and proud of it! (For another 5 years...getting old)

New toy has arrived, 1/4 mile in 11.2 sec and only 114hp!

"the preflash light is analyzed by the same evaluative metering system that the camera uses to meter ambient light. This means it meters through the lens"

I think although the flash metering is using the same system it is independant, so the exposure lock would only lock the ambient, the flash should still expose your subject correctly.

I could be wrong, but best way to find out is to give it a go
Bought a flash yet?

haha not yet will be going for the 580 though...

if you have spare 10min you can try it out hahaha or has anybody else tried it??

__________________
Member of the 18-35 club and proud of it! (For another 5 years...getting old)

New toy has arrived, 1/4 mile in 11.2 sec and only 114hp!