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focal length

American  

noun

Optics.

  1. the distance from a focal point of a lens or mirror to the corresponding principal plane. f

  2. the distance between an object lens and its corresponding focal plane in a telescope.

focal length British  

noun

  1. the distance from the focal point of a lens or mirror to the reflecting surface of the mirror or the centre point of the lens

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
focal length Scientific  
/ fō′kəl /

  1. The distance between the optical center of a lens or mirror to its focal point.

focal length Cultural  

  1. The distance between the focal point of a lens (that is, the point at which the lens will focus parallel rays of light) and the lens itself.

Etymology

Origin of focal length

First recorded in 1745–55

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.