“…For any image-forming system [3][4][5][6][7][8], we consider cartesian coordinates (x,Z) and (x 0 ,Z 0 ) in the object and image planes, cartesian coordinates (X, Y) and (X 0 , Y 0 ) in the reference spheres at the entrance and exit pupils respectively, and, at image space, we take into account a convenient axis termed z 0 (Fig. 1).…”