The aperture is the opening of the lens that determines the volume of light that is let through to expose the film. The aperture is variable and the size is indicated by a series of numbers known as f/stops or f-numbers, which are found on the aperture ring of the lens.
The aperture is controlled by the diaphragm, and their relationship can be compared to that of the iris and the pupil of the eye. The iris (diaphragm) controls the size of the pupil (aperture). In a dimly lit room, the pupil needs to be wide open to allow lots of light through to register an image; the opposite applies in bright light, when it has to shrink to minimize the intensity of light.
