Almost every lens has a more or less round “aperture”, the opening that lets light through to the sensor or film. Most modern lenses have aperture blades that open and close to change the size of the opening. You can see the aperture blades in this photo. Larger apertures obviously let in more light in a given period of time and smaller apertures let in less light in the same period of time. The size of the aperture opening is one factor that determines the exposure. Just as important, the size of the aperture helps determine the depth of field in each image.
