AF in the Dark!!! Do you use video light to focus??

Yuri, by "dragging" the shutter at a lower speed, you allow some of the ambient background light to creep into the exposure, avoiding the picture in the coal mine shot. Your choice of aperture with flash will control depth of field, amount of brightness on the subject, and also influence background illumination to a degree.

I'd most likely keep the shutter speed at 50-80, maybe 30 if the subject is sitting to avoid having blured pictures. Maybe few slower speeds during the dance for some effect! I'm gonna play around with my flash once I get my second one and my trigers to get familiar with it.

You guys are right on keeping it simple for the first few gigs... I need to get comfortable shooting and getting some good angles, rather than running around setting and resetting my second flash. But for the reception, I think I'll have it with me and set it up pointed at the couples table to get some more light..

I'm about to put my son to sleep and gonna try some shots indoors with only few laps and flash

Thanks guys!

Just to add a little more information on the basics, assuming the flash automagically lights the main subject properly, F4 at 1/60th will have the background the same brightness as 1/30th at F5.6. The only significant difference will be depth of field. It's basic exposure, but how you use it is an artistic choice. Stu also uses full frame, so he will have to stop down a little more than I would with a 40D to get the same depth of field.

Good additional commentary Tim - thanks. Not shooting crop I neglect to think of those issues.

Yuri, you are correct in stating that the slower shutter speed can increase the possibility of "blur action.".... been there done that, sometimes with very pleasing results, and sometimes not. The flash duration may be as slow as 1/500 - don't really know the Canon spec on that - but that will give the effect of a 1/500 second shutter speed. Of course if there is sufficient ambient light due to your slow shutter speed, or existing lamps, you will get "comet tail" blurring on fast moving limbs, people, etc.

This can result in some very interesting dance floor effects with band lighting, smoke, spotlights, etc.

tim wrote in post #6455932
AF beam on the 550/580 will help, and the 40D focuses great in low light. Don't worry about it, but if you have trouble focus on an area with contrast, like where a black suit meets a white shirt.
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perfect response.