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focal ratioThe ratio f /d of the focal length, f , of a reflecting surface or refracting medium to its effective diameter, d , i.e. to its aperture. The numerical value of the ratio (often called the f/number) is usually written f/4 or f:4 for a ratio of 4, say. The reciprocal of the focal ratio (d /f) is the aperture ratio. The limiting magnitude – i.e., the apparent brightness of the faintest detectable star – depends on the focal ratio of a telescope: for telescopes used under the same observing conditions, the larger the ratio the fainter the limit; if photographs are being taken, however, the larger the ratio the longer the necessary exposure time.
focal ratio[¦fō·kəl ′rā·shō]