Basic Optical Principles
Plus Lens as Prism

A plus lens may be perceived as a series of prisms placed base to base. Like a minus lens, prism is present everywhere except at the optical center which is the point at which the prism bases come together. When no prism is prescribed, the optical centers should be positioned directly in front of the pupils as in figure B above. When the optical centers are positioned at some other point, prism, either wanted or unwanted, is induced. By studying the examples on this and the previous page, it can be seen that prescribed prism may be intentionally induced simply by manipulating the position of the optical center of the lens relative to the position of the pupil.