Basic Tips for Shooting in Low Light
A few thoughts on shooting in low light situations: To begin with you will need to use a high ISO. Most cameras' lowest ISO is the native (default). This is 100 ISO on Canon and usually 200 on Nikon. You may need to set yours as high as 1600 or 3200 ISO to capture the shot. Do some test shooting before your important shoot.
Shoot images at all of the common ISOs (100, 200, 400, 800, 1600, 3200, 6400). Download your images and compare them. Decide at which ISO your image quality becomes objectionable.
Use a lens with the widest aperture available. Fast lenses have a wide open aperture of F/1.4, F/2.0 or F/2.8. Slower lenses come in F/4, or F/5.6. The widest aperture can be seen on the lens itself. The lens below is a fast lens with an aperture of F/2.8 (circled in red). The faster the lens, the easier it is to focus. Using wider apertures will allow faster shutter speeds.
The wider the focal length lens, the slower the shutter you can use. So if you use a 24mm or 35mm you can shoot with shutter speeds of 1/30th or or 1/40th of a second. This comes from the Hand-Holding Rule which suggests that to find your Shutter Speed, take your focal length and put a one over it. This will ensure a setting that will provide reasonable speed to combat the camera shake that is associated with hand-holding a camera. I usually add a little extra speed.
So this means your shutter speeds for the following focal lengths would be:
If you are using a camera that does not use a full-frame sensor (most, these days) you should go approximately a stop-and-a-half faster. So:
Use a tripod if necessary/possible. This will alleviate the need to hand-hold your camera. This also means you can use slower ISOs for better image quality.
Use A Flash
Experiment at home first. Simulate the conditions under which you will be shooting. Buy an off-camera flash cord so that you can take your flash off the camera and still have it connected. This will provide more options with your lighting. A typical technique is to hold your flash off to the side, at about half and arms length. Point the flash back towards your subject. This can give the illusion that flash was not used. Experiment with different flash positions.
Purchase A Flash Modifier
These modify the flash in some way, such as making it bigger or bouncing it. Modifiers that make your flash a larger light source will produce more pleasing photographs.