Aplanatic Lens

This GGB shows the action of a spherical lens completely eliminating spherical aberration for two special points situated on the axis. If the refractive index of the material of the lens is 'n' and the radius of the lens is 'R', then if object is put at a distance 'R/n', the image will be formed at a distance 'nR' at a distance.
These points at 'R/n' and 'nR' are conjugate points, i.e., these points are interchangeable. Try interchanging these points ....