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relative aperture

The ratio of the diameter of the entrance pupil in an optical system to the equivalent focal length of that system. The relative aperture is denoted as a fraction in which f, the equivalent focal length, is the numerator, and it is symbolized by f/ followed by a numerical value. The ratio f/2 indicates that the diameter of the entrance pupil is equal to one half the equivalent focal length. See f number; lens speed.
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