The aperture is the physical lens opening on your camera. It is adjusted to open and close in f-stop increments, allowing more or less light in with a larger or smaller opening. The iris diaphragm inside the lens of your camera is adjusted to alter the quantity of light that will reach your film. Each aperture opening, or f-stop, lets in twice as much light as the smaller f-stop before it, and half as much light as the larger f-stop after it. For example, f5.6 will let in twice as much light as f8, and half as much light as f4.