Definition of relative aperture
: a measure of the angle of the cone of light rays from an object that traverse an optical system specifically : the effective aperture of a camera lens expressed as a fraction of its focal length with the symbol f being used instead of 1 as the numerator or expressed as the ratio of the effective aperture to the focal length a lens having an effective aperture that is 1/45 of its focal length has a relative aperture of f/4.5 or 1:4.5
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