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Webster 1913 Edition
Focal
Fo′cal
,Adj.
[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.
Webster 1828 Edition
Focal
FO'CAL
,Adj.