It is simple and relatively cheap to make lenses whose surfaces are parts of a plane or parts of a sphere. Such lenses do not focus light perfectly; this lack of focus is spherical aberration. This graphic from a Wikipedia article schematically illustrates how the light rays fail to converge (lower half) compared to a hypothetical perfect lens (upper half).

Spherical aberration is especially prominent in large, fast (bright) lenses. This lack of focus shows up as various forms of fuzziness. The problem can be corrected by placing other lenses in front of and behind the lens. It can also be corrected by changing the shape of the lens surfaces (making them aspherical), but that tends to be more difficult and expensive to carry out: glass spherical shapes are simple to make and measure; aspherical ones are not.

When additional lens elements are added as correction, they increase the amount of flare and reduce the contrast of the photograph, as well as adding to the size and weight of the lens. They might also alter the colors a little. Therefore, aspherical elements typically signal a lens that is crafted to produce contrasty, colorful images and to be lighter and easier to use. They do not in themselves assure high quality, because lenses can exhibit other problems besides spherical aberration. For instance, small aspherical lenses can be molded of plastic and routinely appear in cheap photo equipment. However, a large glass aspherical lens is much more expensive to produce and usually reserved for better lenses.