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relative aperture

noun, Optics.
1.
the ratio of the diameter of a lens, especially a camera lens, to the focal length; the reciprocal of the f number or focal ratio of the lens.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for relative aperture

relative aperture

noun
1.
(photog) the ratio of the equivalent focal length of a lens to the effective aperture of the lens; written as f/n, f:n, or fn, where n is the numerical value of this ratio and is equivalent to the f-number
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Encyclopedia Article for relative aperture

f-number

the measure of the light-gathering power of an optical system. It is expressed in different ways according to the instrument involved. The relative aperture for a microscope is called the numerical aperture (NA) and is equal to the sine of half the angle subtended by the aperture at an object point times the index of refraction of the medium between the object and the objective lens. For binoculars, telescopes, and photographic lenses in which the object may be distant, the relative aperture is taken as the ratio of focal length of the objective to the diameter of the entrance pupil. The relative aperture of a camera lens is sometimes expressed as a simple ratio-e.g., 1:4.5-or more commonly as its f-number, f/4.5. In either case, a lens of 180-millimetre focal length set at this relative aperture would have a pupil diameter (effectively, the lens diaphragm opening) of 40 mm.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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