Lens Form & Analysis
Spheres

Although it is useful to think of lenses as prisms base to base or apex to apex, in reality lenses are not angular in shape at all, rather they consist of curves. The basic curve upon which most ophthalmic lenses are shaped is the sphere. A sphere is a three dimensional geometric shape such as a round ball. As we know ophthalmic lenses are not round balls, however the curve on lenses can be in fact, part of a sphere. Figure D above illustrates a sphere or ball in which a section has been cut off. The curve of the cut off section is a convex curve. Figure B shows a curve which would result if we were to take an �impression� of the sphere and its shape is concave. In optics, sign conventions have been established in which a convex curve is assigned a plus (+) sign, while concave surfaces are given a minus (�) sign. If a surface is perfectly flat it contains no curve at all and is called a �plane� or plano surface.