I want to start taking portraits outside at night. I read that rear curtain flash is best for this.
I have a Sony a700 and when I choose "rear sync" for the flash mode the pop-up flash is activated twice - once at the beginning of the exposure and again at the end, any ideas on how I can stop the flash activating at the beginning of the exposure in order to give me "true" rear sync?
I don't know for certain with Sony cameras, but I imagine that the first flash actually fires before the shutter opens and is a pre-flash to measure the flash strength so that it can be set for the exposure proper. You notice it more easily with rear sync because there's the length of the exposure between the two flashes.
If you want to capture background detail lit by whatever ambient light there is, slow or rear sync is the way to go. Moving objects that are patly ambient & partly flash lit tend to look better with rear sync.
Rear Sync on nikon is just one flash in time with shutter - not quite sure why sony would use 2?
Marcus, I don't know if you're not counting the preflash sequence, or if you're using some non-default flash mode like manual or FV-lock, or you simply haven't noticed it, but Nikon do use 2 flashes for the i-TTL system, a burst before the shutter opens to measure the intensity required during the exposure and the flash when the shutter is open. Try rear sync with a 1/2 second exposure. You will see the two distinct flashes.
I don't know about Sony, either, but this is what I was basing my reply upon. Seems like Peter isn't interested any more, and that no Sony users can contribute a better explanation, so we may never know.