An Etymological Dictionary of Astronomy and Astrophysics

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فرهنگ ریشه‌شناختی اخترشناسی-اخترفیزیک

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nominal focal length
  درازای ِ کانونی ِ نامنال  
derâzâ-ye kânuni-ye nâmenâl
Fr.: distance focale nominale  

An approximate value of the → focal length, used for the classification of lenses, mirrors, or cameras, as opposed to the measured one.

See also: → nominal; → focal; → length.