Geoff wrote:
Alan Browne wrote:
Probably pre-flash flash metering.
1) Did the images look okay?
2) If you determine the flash power seperately (incident flash meter)
and you can set your (attachement) flash power manually, then you can
get rid of the pre-flash.
Thanks for both your replies Alan. The images did look ok but I wasn't
aiming for a particular effect really, more just experimenting and was
confused when I saw the first flash. With the camera set in normal front
curtain mode, I thought the flash did do a quick pre-flash that was
barely detectable. In fact I guess that as the shutter is open for
longer and it's set to rear curtain, that just means that the pre-flash
was noticable in my case last night. Hmmm....Ok I think I get it now! So
is the first flash actually taking place before the shutter is open? I
didn't thnik it was but maybe I just assumed that.
Yes. In film cameras with TTL flash, flash is controlled by
continuously measuring the flash light being received by the film.
There are sensors in the film chamber that catch the light reflected off
of the film, integrate that and then shutoff the flash when the right
amount of flash light is received.
This is not possible with digital cameras as there is no useful
reflection from the sensor. Off-The-Film (OTF) doesn't work.
So, pre-flash is used. It fires before the mirror lifts and the
metering system in the prism assembly (used for available light
meterering) are used to measure the pre-flash light return from the
subject area. This is often integrated with the focus distance of the
lens. The flash power is calculated and then that "dose" is used during
the shot. The usual problems with depending on subject reflectivity,
actual subject position v. sensor position, etc. apply.
I can't honestly say if it's causing a problem with the images or not,
as I'm just trying to understand things at this point rather than
guessing, so I can then plan exactly what images I want, then set the
camera up as I think would be right for the situation. I never like
guessing because I want to know why I'm getting the effect I'm getting. 
Post examples of what you're getting.
Cheers,
Alan.
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