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Photonics Dictionary

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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