With 40D & 580EX:
My thumb gets worn out hitting the AE-Lock button to get the flash to pre-fire before releasing the shutter.
Is there a function option the get the 580ex to pre-fire every time I press the shutter half way?
I find without it I get inconsistent exposures. If I press the AE I get a keeper every time.
I could use some setup or button layout scenerios.
The preflash occurs just before the mirror goes up.
It's not the timing that's making the difference (well, probably not). It's that when you press the * button you're using a Spot metering for the flash, whereas when you don't use the * button you're using Evaluative metering for the flash.
With the * button the camera assumes that whatever's in the center of the viewfinder is your subject, whereas with Evaluative metering the camera assumes that whatever gives the brightest flash return is your subject. That's usually the closest object of any size, although something bright white that's not much farther away can sometimes be pegged as the subject.
Try setting the E-TTL II mode to "Average" (page 104 of the 40D manual). This will make the camera forget about looking for a subject and instead meter the entire scene.
