580EXII firing twice with ETTL and second curtain flash

[GoingUp]

Taking a few last minute pics of my assistant for tomorrows wedding to review some last minute flash camera settings.

Now when I have my 40D in AV mode, and the flash set to ETTL, it shoots a flash at the beginning of the shutter open and at the close. Previously it only shot it at the end of the shutter close.

I've never seen this before. I've tried in both dim rooms and outside in more light.

Thanks.

 

[angry hampster]

What to you have the camera set to in the custom functions?

 

[Deadtrees]

I think you're experiencing the difference between 1st and 2nd Curtain Sync.

 

[GoingUp]

Originally posted by: Deadtrees
I think you're experiencing the difference between 1st and 2nd Curtain Sync.

Except the flash literally fires twice.

 

[Zenmervolt]

All DSLRs use a pre-flash for TTL metering. Unlike film, which was semi-reflective and allowed the flash to be metered by a photo diode that read the light reflected by the film surface, a DSLR's sensor does not reflect light back. So the camera needs to fire a "test" flash of known strength to meter off of.

Wikipedia article on Canon's specific implementation: http://en.wikipedia.org/wiki/Speedlite

ZV

 

[Deadtrees]

Originally posted by: Gobadgrs

Originally posted by: Deadtrees
I think you're experiencing the difference between 1st and 2nd Curtain Sync.

Except the flash literally fires twice.

Because it's E-TTL.

 

[soydios]

The flash always fires twice: the metering preflash and the exposure flash after the shutter opens. With front-curtain flash sync you barely notice the time delay between the metering and main flashes. But with rear-curtain sync you have enough time delay to notice the two flashes.