Spherical lenses, also known as singlets, are transparent optical components consisting of one or more pieces of optical glass with surfaces curved to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.  

There are seven main spherical lens forms, or shapes, that determine the imaging characteristics of the lens.  They are plano-convex, plano-concave, convex-convex, concave-concave, meniscus, aspheric, and ball.  Plano-convex lenses have a positive focal length, which makes them ideal for collecting and focusing light for many imaging applications.  Plano-concave lenses have a negative focal length and are used for image reduction or to spread light.  Convex-convex lenses have a positive focal length and are useful for 1:1 imaging and in multi-element systems. Also known as biconvex or equiconvex.  Concave-concave lenses have negative focal length, and are used for image reduction and to spread light. Also known as biconcave or equiconcave.  Meniscus lenses can increase the numerical aperture of a positive lens assembly, without an undue increase in the aberrations.  Aspheric lenses compensate for spherical aberration and are used primarily for their light gathering ability.  Ball lenses are used to provide short focal lengths for use with collimated light.  They are often used in fiber coupling applications.  While the above seven are the most common types, there are other, rare styles of spherical lenses available.