Single Picture Explains How Aperture, Shutter Speed, and ISO Work In Photography

Sorry these are posting out of order, but one last factor that isn't covered here is the focal length of your lens. In low light with potentially slow shutter speeds, the steadiness of you hand makes an impact. A general rule is that your shutter speed should not be slower that 1/[focal length], so for an 85mm lens it would be 1/85. If you're up close with a wide angle lens, you'll have more flexibility. If you're using a longer focal length, at a certain point the fastest shutter speed you can use might be too slow to prevent blur created by camera shake.

Lastly: Shutter speed is your final choice. If the lighting is pretty consistent, take a few test shots, and go with the one that achieves the exposure that looks best to you. If it turns out that you're working with a slower speed that can freeze even slight motion, you'll have to be choiceful in your timing and shoot only where you sense there's about to be a moment of stillness; or, when there's a moment of fluid motion that you are specifically looking to blur (such as a drummer keeping his head still while his hands are moving wildly). If the lighting is highly varied and constantly changing, you'll be having to constantly adjust your shutter speed. You'll have to use whetever setup feel easiest to you. It could be setting your camera to aperture priority so that the camera is adjusting the shutter speed for you on the fly. Just have to remember that you can freeze more motion when the light is brighter, and when it's darker to only shoot when subjects are still.

1st: start with the highest ISO you're willing to tolerate in terms of grain/noise. The more noise you're willing to accept, the greater flexibility you'll have in terms of exposure. Next, shoot with you lens's widest aperture: f/1.2, 1.4, or 1.8 at least. If you don't have a lens that can achieve these apertures, you might want to consider on that does. The tradeoff here is that you're working with a shallow depth of field, so you'll just have to get very fast and very precise with focusing. Sometimes a certain autofucus setup can be faster, sometimes focusing the lens manually could be fastest.