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n. Abbr. f
The distance of a lens or mirror to its focal point. Also called focal distance, focus.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(General Physics) 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 and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
the distance from a focal point of a lens or mirror to the corresponding principal plane.Also called fo′cal dis`tance.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
The distance from the surface of a lens or mirror to the point where the light rays converge to form an image (the point of focus).
The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.