The focusing of rays from a point source on the optical axis into points whose distance from the focal plane varies as the angle at which the rays entered the optical system, with rays entering at a small angle to the optical axis focusing closer to the focal plane than rays entering at a larger angle. The envelope of the converging rays is called the caustic of the optical system. The amount of spherical aberration varies with the location of the source and for any particular source is approximately proportional to the square of the distance of the outermost rays from the optical axis.