Definitions for focal length
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Random House Webster's College Dictionary
fo′cal length`(n.)
the distance from a focal point of a lens or mirror to the corresponding principal plane.
Category: Optics
Princeton's WordNet
focal distance, focal length(noun)
the distance from a lens to its focus
Wiktionary
focal length(Noun)
The distance at which something (e.g. a lens, mirror etc.) is in focus; in air it is the distance along the optical axis from a "principal plane" of the lens to one of the lens's focal points.
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"focal length." Definitions.net. STANDS4 LLC, 2013. Web. 19 May 2013. <http://www.definitions.net/definition/focal length>.