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US number

 

photography A measure of aperture size, hence of light-gathering capacity. Unlike the more familiar f number, US number is inversely proportional to area rather than to diameter. The scale starts with 1 equalling f4 (technically f/4), then proceeds with whole numbers for the familiar British f values. See also film speed.

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Measures and Units. A Dictionary of Weights, Measures, and Units. Copyright © Donald Fenna 2002, 2004. All rights reserved.  Read more