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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.
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.
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