rear curtain sync on canon XSi question

[syphlix]

when i have it set on rear curtain, the flash pops at the beginning of the exposure AND the end.

is this how it's supposed to work? i kinda assumed it was only supposed to pop at the end... does it make a difference?

 

[Big Mike]

The first one is only for metering.

Canon uses something called 'E-TTL' for their flash metering. It works by popping the flash before the shutter opens, reading the light that bounces off of the subject and back into the lens, then using that information to determine how much flash power to use when the shutter actually opens...which happens right after.
It can all happen so fast that most people don't even realize that it was more than one flash.

Now, if you set the flash to rear curtain sync, the flash fires at the end of the exposure, rather than at the start. The metering pop still has to fire before the exposure...so the longer the exposure, the more time between the flash pops.

So yes, that is how it's supposed to work.
It's really only a problem when you're shooting people and they think the photo is over when the flash pops the first time...and start to move/look away.

 

[syphlix]

ah that makes sense

thanks mike