In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the focal length of the lens. Lenses whose thickness is not negligible are sometimes called thick lenses to emphasize that their thickness is not being neglected.

The thin lens approximation ignores the thickness of lenses and simplifies ray tracing calculations. It is often combined with the paraxial approximation in techniques such as ray transfer matrix analysis.

Lenses