THE APERTURE IS the opening in the lens that lets light through to the sensor in the camera. The term aperture is also used to describe the size of the opening in the lens, which is represented by an f/stop number. Since the f/stop is a fraction, the smaller the denominator, the larger the opening, as seen in Figure 3.1.
When you’re shooting images in low light and need to use a fast shutter speed to freeze the action—at a concert or sporting event, for example—you will want to use as wide of an aperture as possible. For example, I shoot most of my concert photos at f/2.8, which is a very wide aperture.