The opening of a lens that controls the amount of light entering a camera is called the aperture. Inside many photographic lenses, a diaphragm of overlapping blades changes the aperture’s size, much like the iris controls the pupil of the human eye.
A wide aperture allows more light to reach the film or image sensor, which can help in dim conditions or permit a faster shutter speed. A narrow aperture admits less light and often produces a greater depth of field, keeping more of the foreground and background acceptably sharp.
Photographers describe aperture using f-numbers such as f/2.8, f/5.6, and f/16. The numbering can seem backwards: a smaller f-number represents a wider opening, while a larger f-number represents a narrower one. Each full stop changes the amount of light by a factor of two.
Aperture is one part of the exposure triangle, alongside shutter speed and ISO sensitivity. It is also commonly confused with the diaphragm itself: the diaphragm is the adjustable mechanism, while the aperture is the opening that determines how much light passes through the lens.