2nd sync preflash????

[Wayne Photography]

Hi guys...
I have a 580 ex and a 40d...question is this...

When I set the flash to 2nd sync flash (in m mode)with a longish exposure,it first gives a a pre flash then 2nd syncs after the set time exposure....surely it should not fire first on pressing the shutter.....just cant understand this...2 friends of mine also with 580ex's experience the same thing with different cameras ,namely a 5d and a 350d.....

Can someone please enlighten me and perhaps explain how to stop it doing this...thanx
Wayne.

[aacmckay]

The flash will fire before the shutter opens for TTL metering. So the sequence when you press the shutter the flash fires and camera meters it. First curtain opens for the length of time determined by the shutter speed. Just before the second curtain closes the flash fires at the determined power, then the shutter closes. It is perfectly normal for the flash to preflash when the shutter is pressed in second curtain sync.

[bwolford]

If you don't want this preflash, you need to go to manual mode for the camera and the flash.

[aacmckay]

I'm not sure that the camera necessarily has to be in manual. But putting the flash in manual would stop the TTL metering preflash.

[bwolford]

You don't, that's correct. I have just developed the habit, after fearing M on the camera and I don't think I'll go back to any automated mode, with the possible exception of Tv or Av in certain circumstances. Most often without the flash.