Cardinal points of an optical system

Summary

The cardinal points of a rotationally symmetric optical system are six points on its axis: two focal points, two principal points and two nodal points. In the paraxial approximation, they determine how the system forms images. Any such system, however many lenses it has, then acts like a single thin lens, once distances are measured from its two principal planes. They reduce the design and analysis of compound lenses to a few numbers. See the definition.