High Quality Content by WIKIPEDIA articles! In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of an ideal, rotationally symmetric, focal, optical system. For ideal systems, the basic imaging properties (e.g., image size, location, and orientation) are completely determined by the locations of the cardinal points; in fact only four points are necessary: the focal points and either the principal or nodal points. However, the only ideal system that has been achieved in practice is the plane mirror. Consequently, the practical utility of cardinal points would be very limited. However, the cardinal points are widely used to approximate the behavior of real rotationally symmetric, focal, systems.