flash 2nd curtain sync

[perfectpixel]

Hi,
I am fooling around with the 10D, a 420EX flash and 2nd curtain sync doing long exposures (couple of seconds) of an illuminated moving object.
Here's my question: When I release the shutter, the flash gives a pulse of light, and then at the end of the exposure time I get the expected, nuch stronger flash. What is that first pulse? I thought 2nd curtain syc only fired at the end of the eposure :?:

[RichardtheSane]

The first pulse it the ETTL preflash, which measured the scene so the second pulse is the correct output.
It actually fires before the shutter opens too :)

[scottbergerphoto]

Richard is correct. In second curtain sync, the flash fires at the end of the exposure. When you depress the shutter release button a preflash fires before the shutter opens. Then the shutter opens and usually the flash fires instantaneously so you don't see the two flashes. In 2nd curtain sync, you see the 2 flashes due to the delay. You can create the same delay using flash with mirror lock up or using FEL. You won't get the visual effects of 2nd curtain sync though.
Scott

[RichardtheSane]

Nice extra bit of an explanation scott :)

[scottbergerphoto]

Nice extra bit of an explanation scott :)
My pleasure. Thank you.
Scott