GNU Collaborative International Dictionary of English

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1.       
   Focal (?), a. [Cf. F. focal. See Focus.] Belonging to,or concerning, a focus; as, a focal point.
   
   [1913 Webster]
   
   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.
   
   [1913 Webster]