f-num·ber (?f?n?m?b?r)n. The ratio of the focal length of a lens or lens system to the effective diameter of its aperture. Also called f-stop.[f(ocal length) + number.]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.f-number or f numbern (Photography) photog the numerical value of the relative aperture. If the relative aperture is f8, 8 is the f-number and indicates that the focal length of the lens is 8 times the size of the lens aperture. See also T-numberCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014f-number (??f?n?m b?r) n. a number corresponding to the ratio of the focal length to the diameter of a lens system, esp. a camera lens. Symbol: f/ [1890?95] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.