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equivalent focal length

(optics) the ratio of the size of an image of a small distant object near the optical axis to the angular distance of the object in radians
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Examples from the Web for equivalent focal length
Historical Examples
  • The equivalent focal length of a negative or Huygens eyepiece is equal to half the focal length of the larger or field lens.

    Pleasures of the telescope Garrett Serviss
  • When the simple67 expression "focal length" is used, it always refers to the equivalent focal length.

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