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The lens focal length divided by effective diameter of the aperture gives the f/number that's used to indicate the aperture value. Each full f/number, also called a stop or f-stop, halves or doubles image brightness and some lenses can also be controlled in half or third steps. The most common f/numbers are 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22. They are usually preceded by an "f". The larger the f-number, the smaller the lens opening. In the aforementioned series of numbers, f/1.4 is the largest opening and f/22 the smallest. The smaller stops (larger f numbers) give the greater depth of field in a photograph, and vice versa.
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A hole in the lens that adjusts in diameter, similar to the way the pupil of an eye works. This controls the amount of light reaching the film or CCD to record an image. Every different diameter opening has a number which indicates the size - it equals the focal length of the lens divided by the diameter of the aperture. These numbers are also called stops. See f/number.