fo·cal /ˈfokəl/ 形容詞
  病灶的,局灶的,局部的,焦點的
  focal
  焦點
  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.
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  focal
       adj 1: having or localized centrally at a focus; "focal point";
              "focal infection"
       2: of or relating to a focus; "focal length"