This GGB shows the action of a spherical lens completely eliminating spherical aberration for two[br]special points situated on the axis. If the refractive index of the material of the lens is 'n' and the [br]radius of the lens is 'R', then if object is put at a distance 'R/n', the image will be formed at a [br]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 ....