Chromatic aberrations in lens design

In texts on geometrical optics and lens design usually two types of chromatic aberrations are discussed: longitudinal and transverse. From basic considerations on first order geometrical optics follows that, for an axially symmetric system there are three paraxial constants. Therefore three, instead of two types of chromatic aberrations can be discerned. The third, new, chromatic aberration can be called chromatic pupil aberration. We will describe the consequences of this aberration for the colour correction of optical systems, and show that stable chromatic correction requires the elimination of all three chromatic errors. We will give expressions that can be used in the lay-out of optical systems. In teaching geometrical optics it is necessary to determine the generic aberrations of a system of given symmetry from first principles; our treatment of chromatic aberrations is an example of this necessity.