The merit function operands REAX and REAY can be used to determine the x and y coordinates of an arbitrary ray on any surface, including the image surface. The ray is defined by its normalized field and pupil coordinates. For example, Hx=1, Hy=0, Px=0, Py=0 defines a chief ray in the YZ plane. Together with general math operands, this can be used to calculate the lateral magnification in the X direction, or in any other direction.