All types of lenses
Symmetrical bi–convex and bi–concave lenses are the simplest of lenses to understand. There are very many different lenses.
If one of the surfaces of a convex lens is a plane while the other is convex, the lens is called a plano–convex lens. If one of the surfaces of a convex lens is concave while the other is convex, the lens is called a concavo–convex lens.
Similar names are also used for concave lenses. If both the surfaces of a concave lens are concave, the lens is said to be a biconcave or double concave lens. If one of the surfaces of a concave lens is a plane while the other is concave, the lens is called a plano–concave lens. If one of the surfaces of a concave lens is convex while the other is concave, the lens is called a convex–concave lens.