Before shooting at any venue, if it were me I'd try to go by and do some lighting tests in advance.
You might need to filter the flash's output, to match the color of the type of lighting being used (the color of the bare lighting, without any of the color filtration). You will probably need to set a custom or hand selected color temp in the camera, to keep Auto White Balance from reacting to the colored stage lighting and "correting" them.
Start with your camera on one of the automatic exposure modes, and the flash set to ETTL and FEC set to 0. The camera will try to make the image by the ambient light, recording the stage lighting, and the flash will be fired as FILL, about 1.5 to 1.7 stops underexposure. Dial it up or down as you see fit, to increase or decrease the fill, using FEC.
You can do this manually, too, as described above. Set up the camera alone, without the flash, to record the basic exposure. Just remember that in M mode the camera will want to fire the flash as FULL, so you will need to dial it down 1.5 to 1.7 stops, or maybe a little more or a little less.
When using the camera in M, you could use the 1/4 power setting of the flash, too... but the result of that will be the same as -2 stops. The problem with using the fractional settings is that it will be full stop increments only (1/2 = 1 stop, 1/4 = 2 stops, 1/8= 3 stops, etc.)
You very likely don't want to use bounce flash in a case like this. You will need direct flash. You likely shouldn't try to diffuse the fill flash, either. Bounce or diffusion will simply force the flash to fire more powerfully and recycle more slowly. Bounce light also can cause unwanted shadoes, or if bounced off too high a ceiling or a ceiling that's too dark colored can be underpowered or lost completely, or can cause a tint if the ceiling is any color other than white.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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