Basic Optical Principles
Minus Lens as Prism

As mentioned earlier, a minus lens may be perceived as a series of prisms placed apex to apex. Given this information, it can be understood that prism is present everywhere in a lens except for the point at which the apices meet. This point is called the optical center. If no prism is prescribed, the optical centers should be positioned directly in front of the pupils as in figure B above. When the optical center is positioned at some other point, prism, either wanted or unwanted, is induced. The direction of the base will depend upon where the OC is positioned relative to the pupil as demonstrated by the other examples on this page.