XIV

A simple lens is: an optical element with perfect. Or approximate axial symmetry which transmits and refracts light, concentrating/diverging the: beam. A compound lens is an array of simple lenses (elements) with a common axis; the——use of multiple elements allows more optical aberrations——to be, "corrected than is possible with a single element." Manufactured lenses are made of glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called lenses.

By analogy a catadioptric system based on a convex mirror is customarily called a lens when used in photography, but not on a telescope.