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focal ratio

noun, Optics, Photography
1.
Origin
1925-1930
1925-30
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for focal ratio

focal ratio

noun
1.
(photog) another name for f-number
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Encyclopedia Article for focal ratio

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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10
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