(On board) flash fires twice

[ecniv]

Hi,

Probably asked before, sorry didn't search.

Had the d300 set up for rear sync, trying to capture my car with the foggy/misty background last week. Everytime I focussed as soo as I fully depressed I got two flashes, one at the start and one at the end. This produced a shake (hand held).

Is there a reason why the flash fires first time? Can I stop it?

 

[JonathanRyan]

Is there a reason why the flash fires first time?

Yes. It's how it meters.

It always does this in TTL but with a regular exposure the flashes are so close together they are indistinguishable. In rear curtain with slow shutter speed metering flash is at the start (before shutter opens) lighting flash at the end.

Can I stop it?

Put your flash in manual. Maybe automatic too (can't remember - too much shooting in the cold today...)

 

[ecniv]

Thanks