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focal ratio
The ratio of the focal length (F) of a mirror or lens to its diameter (D) expressed as a number, f/# = F/D. Borrowing the language of photography, small focal ratios, below about f/6, are said to be fast and result in a brighter image for a given aperture. Large focal ratios, equal to or above about f/8, are said to be slow.
Related category
• OPTICS AND OPTICAL PHENOMENA
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