In photography, the aperture defines the size of the opening in the lens, which in advanced cameras can be adjusted to control the amount of light reaching the film or digital sensor (CCD or CMOS). In combination with variation of the shutter speed, this will regulate the photograph's degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure a sufficient exposure to light, just as a slow shutter speed will typically require a smaller aperture to prevent excessive exposure to light.

The word "aperture" means an opening, from the Latin apere, to open.

A device called a diaphragm controls the aperture. The diaphragm can be considered to function much like the pupil of the eye—it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture, the greater the distance from the plane of focus the subject matter may be while still appearing in focus.

Aperture is usually measured in f-numbers. A lens will have a set of "f-stops" that represent doublings in the amount of light let through the aperture. A lower f-stop number denotes a greater aperture opening which allows more light to reach the film. A typical standard lens will have an f-stop range from f/16 (small aperture) to f/2 (large aperture). Professional lenses can have f-stops as low as f/1.0 (very large aperture). These are known as "fast" lenses because they allow much more light to reach the film and therefore reduce the exposure time of the film. These lenses are favored especially by photojournalists who often work in dim light, have no opportunity to introduce supplementary lighting, and capture fast breaking events. This is true for prime lenses (lenses which have a fixed focal length), but isn't for zoom lenses.

Zoom lenses typically go from f/2.8 to f/6.3. A very fast zoom lens will be constant f/2.8, which means the aperture will stay the same throughout the zoom range. A normal zoom will be a constant f/4, and a consumer zoom will typically be with a variable diaphragm, normally being something along the lines of f/4.5 to f/5.6, or even f/4.5 to f/6.3 (rare). There are a few exceptions to this rule, as even high quality hyperzooms often have as slow of an aperture as f/5.6 throughout the whole zoom range. Such is the case with most lens which have more than 4x zoom range, like a 100-400mm f/5.6.

The reason for consumer zooms to have a variable aperture is that the aperture is calculated by dividing the focal length by the diameter of the actual diaphragm. So if you have for example a 75-300mm, a physically bigger diaphragm will be needed at 300mm than at 75mm, but the aperture will remain the same, as you need more light as the focal length increases.

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• Controlling Exposure