Kennedy McEwen wrote:
I don't think it was that because that normally happens immediately
before the main flash and is barely detectable.

It normally happens immediately before the main flash in front curtain
sync, but it has to happen before the mirror goes up, so it will be a
long time before the main flash with rear curtain sync, and will appear
right at the start of the exposure (actually fractionally before the
exposure commences).

Also, this first flash is happening once the shutter is open, so ruins
the effect I'm trying to acheive by having rear curtain set.

Are you sure it is happening *after* the shutter is open, and not just
before it opens? In other words, does the first flash appear on the
image, resulting in two sharp images of moving objects, one at the
beginning of the movement trail and one at the end?

If it does then you either aren't properly setting rear curtain sync or
there is a fault with your camera. :-(


I've only just posted something that your post has just backed up, so it
all makes sense to me now. I kinda worked out what was going on as I was
writing my last post, and you've assured me I was right. As you and Alan
suggest, the first flash isn't going off before the shutter opens, it
can't be. I assumed it was for some reason but I guess that isn't the
case. In fact now I'm aware of that it also helps me understand why some
of my test images weren't exposed correctly (I was messing about zooming
out while the shutter was open to get some nice streaking effects around
the main image).

Thanks for your help,
Geoff.