If you're using ISO800 and you're already shooting wide open at those slow shutter speeds your only option is flash or a higher ISO.
I don't know if I'd recommend a higher ISO on the 10D. It may be too noisy, but give it a go and see how it works out.
Have you been using the shutter drag technique with your flash shots.?
- 'Dragging' the shutter
Having lights glowing in the background generally looks better than a black background or flash shadow. If you are doing groups and excluding daylight from the windows; set camera aperture to say, f8, and allow the shutter to drag. (Typically 1/15 to 1/8 of a second is about right). NB Use the tripod and expose with the flash.
NB The flash helps to keep the foreground sharp of you're hand holding the camera - so you can drop the shutter speed on a standard lens to1/30 of a second if you're of a fairly steady hand, to help provide a warm flattering background effect with the lights 'switched on'.
You might get something interesting that way. Gives a kind of ghosting effect to anything that moves after the flash fires while giving a sharp image at the same time. Don't use program mode. Use AV or manual.
Blur in these kind of shots adds to the feel IMO.
Here's an example of shutter drag. Flash fired wwhile the shutter stayed open for a few more seconds.
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