| 单词 | focal ratio |
| 释义 | > as lemmasfocal ratio focal ratio n. †(a) Mathematics the ratio of the distances from a given point on a conic section to its directrix and to one of its foci (obsolete rare); (b) the ratio of the focal length of a lens, telescope, etc., to the effective diameter of the aperture, used typically to express its light-gathering power; = f-number n. at F n. Additions. ΚΠ 1835 S. W. Waud Treat. Algebraical Geom. Contents p. xiv From the position of the directrix and focus, and focal ratio, to find the equation to the curves of the second order. 1883 Eng. Mechanic & World of Sci. 38 259/2 A correction is applied to the focal ratio for removing dispersion, which may possibly improve the achromatism. 1945 Pop. Mech. Jan. 148/2 Such a lens was finally designed by Dr. W. B. Rayton..that gave excellent definition with a focal ratio of f:0.6. 2005 R. B. Thompson & B. F. Thompson Astron. Hacks i. 76 Large refractors usually have short focal ratios to allow reasonable tube lengths of a meter or so. < as lemmas |
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