Tutorial : focal lengths - principal planes - catadioptric system

Basic catadioptric systems are made of a refractive system and a mirror. Light goes first through the refractive system, is then reflected back by the mirror and finally goes through the refractive system a second time in the reverse way.

In its simplest configuration, a catadioptric system is made of a thin lens and a spherical mirror. The whole system is reflective and behaves like a virtual spherical mirror with an equivalent vertex S_eq and an equivalent radius R_eq.

R_eq and the distance x_eq from the lens to S_eq depend on the effective focal length of the lens fi, the mirror radius R and the distance L between the lens and the mirror vertex S_m.