I do a regular stint at a monthly dance with low lighting levels and some colour lighting effects. Not too different to a nightclub, really.

I use a canon 40D and a 580ex II with a Gary Fong lightsphere on it. I use it on ISO 400 and the camera on manual and the flash set to do its own thing (set to 2nd curtain sync).

I find that the output can vary wildly - some are exposed fine, yet some can be anything up to 2 stops underexposed. It seems really random and I can't seem to find one thing that affects it. I've even tried different batteries, removing the "tupperware" and using a stofen instead.

Does anyone else find when they're shooting? Perhaps I'm expecting too much from such difficult lighting conditions? I do shoot in RAW, and can "rescue" most of the images, but it can add a fair amount of noise that I'd prefer not to have, as I am shooting at ISO 400 (I may be imagining things, but I am sure my camera is noisier at 400 ISO than it should be).

The only thing I can think of is the lens not being a constant f/2.8 - it's a Sigma 17-70mm with something like f/2.8-4.5. I know zooming can close down the aperture, but I would have thought the flash would compensate for it.

Anyone have any thoughts?

That does proper exposure for the distance your shooting at, if the subject moves farther away it will be under and if they move closer it will be over exposed.

I stopped using 2nd curtain sync for that reason.

This is because the flash fires twice. The first flash at 1st curtain is a metering flash, so as Zulu says, if the scene changes then the metering will be wrong.

I learned that one the harder way. Fuck it overexpose everything

set your ISO to 100/200-whatever the lowest is(not less than 100)- then raise your shutter speed to at 160 and drop your F-stop . i personally love hotlighting images, but correct lighting with most flashes and limited metering starts with low ISOs

because youre using ettl on your flash and not manual.

wtf?

jef