fo·cal /ˈfokəl/ 形容詞
Fo·cal a. 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.
adj 1: having or localized centrally at a focus; "focal point";
2: of or relating to a focus; "focal length"