Focal
Focal , adjective
[Compare French focal. See Focus.]
Belonging to,or concerning, a focus; as, a focal point.
Collocations (2)
Focal distance, or length, of a lens or mirror (Optics) , 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.