F-stop

Before you know what f-stop you want, you first have to know what it is you are trying to accomplish in the photograph: Do I want to freeze action, or show motion, and consider shutter speed, or, do I need to get a cast of people in focus, or focus selectively, and cater to aperture?

If you're shooting at an f-stop of 2.8, then the lens will take in a lot more light than it would if the f-stop was at 22.