Definitions for focal length
Random House Webster's College Dictionary
the distance from a focal point of a lens or mirror to the corresponding principal plane.
focal distance, focal length(noun)
the distance from a lens to its focus
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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