Where and are the refractive indices and focal length of the m thin lens forming the lens system.

Let us consider a system of two lens, such that =0, ie.

(Petzval condition)

If these two lens are made of same material and have opposite focal length ((one convex and other concave), then such a lens system is free form the field curvature aberration. If the distance between the two lens is d then the combined focal length of the system is given as

                                                                 or       ,