focal ratio

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The ratio of the focal length of a lens or mirror to its aperture, usually expressed as a number, the f/number. The smaller the focal ratio, the smaller the image scale and the brighter the image for a given aperture. Small focal ratios, below about f/6, are termed fast, while those greater than about f/8 are slow, a terminology adopted from photography.

Subjects: Astronomy and Astrophysics.

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