Various types of cardinal points are defined in Gaussian optics for characterizing the optical function of a single optical element or a group of such elements:
- There are the front and back focal points at the intersection of the focal planes with the optical axis.
- The principal points are at the intersections of the principal planes with the optical axis.
- Further, there are the front and back nodal points.
The cardinal points can be calculated from the ABCD matrix, and vice versa. They are useful, for example, when geometrically constructing images of objects.
According to the basic assumptions of Gaussian optics, the rays are considered for the limiting case of small angles against the optical axis. If parallel incoming rays are calculated for a wider range of offsets, for example, will generally not exactly meet in a focal point.