Hypertext Webster Gateway: "focal"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
Focal \Fo"cal\, a. [Cf. F. focal. See {Focus}.]
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.
From WordNet (r) 1.7 (wn)
focal
adj 1: having or localized centrally at a focus; "focal point";
"focal infection"
2: of or relating to a focus; "focal length"
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