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From: Webster's Revised Unabridged Dictionary (1913)

 Fo·cal a.  Belonging to,or concerning, a focus; as, a focal point.
 Focal distance, or length, of a lens or mirror Opt., the distance of the focus from the surface of the lens or mirror, or more exactly, in the case of a lens, from its optical center. --Focal distance of a telescope, the distance of the image of an object from the object glass.
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