Lens

An optical device designed to produce an image on a screen , on a camera film, and in a variety of optical instruments. Also used to converge, diverge or otherwise control light rays in applications not imvolving images. A transparent device for converging or diverging rays of light by refraction. Convex lenses are thicker at the center than the edges; concave lenses are thicker at the edges than at the center.

Lens axis

A line through the center of curvature of a lens.

Lens Flare

Non-image forming light reflected from lens surfaces that degrades the quality of the image. A problem with simple projection lens.