focal length(redirected from Back focal length)
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n. Abbr. f
The distance of a lens or mirror to its focal point. Also called focal distance, focus.
(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
the distance from a focal point of a lens or mirror to the corresponding principal plane.Also called fo′cal dis`tance.
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).