01 a. Belonging to,or concerning, a focus; as, a focal point.
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1.
Belonging to,or concerning, a focus; as, a focal point.
Phrases & compounds
Focal distance —
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.