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focal length

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noun Optics.
the distance from a focal point of a lens or mirror to the corresponding principal plane. Symbol: f
the distance between an object lens and its corresponding focal plane in a telescope.
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Also called focal distance.

Origin of focal length

First recorded in 1745–55
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2022

How to use focal length in a sentence

British Dictionary definitions for focal length

focal length

focal distance


noun
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

Scientific definitions for focal length

focal length
[ fōkəl ]

The distance between the optical center of a lens or mirror to its focal point.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for focal length

focal length

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.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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