ABSTRACT
A lens is made of a material, such as glass, that has a refractive index greater than one. The surfaces are ground and polished until they have the desired shape. Usually the surfaces are spherical in shape because it is very costly to grind aspherical surfaces of high quality, but there are exceptions. Spectacle lenses may have cylindrical shapes superimposed on spherical surfaces to correct for visual astigmatism. Sometimes in the case of lenses to be produced in high quantity, such as certain camera lenses, plastic lenses may be used because the cost of preparing a mold with aspheric surfaces is spread over a large number of lenses.
