<P> More complex are aspheric lenses . These are lenses where one or both surfaces have a shape that is neither spherical nor cylindrical . The more complicated shapes allow such lenses to form images with less aberration than standard simple lenses, but they are more difficult and expensive to produce . </P> <P> Lenses are classified by the curvature of the two optical surfaces . A lens is biconvex (or double convex, or just convex) if both surfaces are convex . If both surfaces have the same radius of curvature, the lens is equiconvex . A lens with two concave surfaces is biconcave (or just concave). If one of the surfaces is flat, the lens is plano - convex or plano - concave depending on the curvature of the other surface . A lens with one convex and one concave side is convex - concave or meniscus . It is this type of lens that is most commonly used in corrective lenses . </P> <P> If the lens is biconvex or plano - convex, a collimated beam of light passing through the lens converges to a spot (a focus) behind the lens . In this case, the lens is called a positive or converging lens . The distance from the lens to the spot is the focal length of the lens, which is commonly abbreviated f in diagrams and equations . An extended hemispherical lens is a special type of plano - convex lens, in which the lens's curved surface is a full hemisphere and the lens is much thicker than the radius of curvature . </P> <Table> <Tr> <Td> </Td> <Td> </Td> </Tr> </Table>

Why convex lens is known as converging lens
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