Parameters
fi1 : effective back focal length of the first lens
fi2 : effective back focal length of the second lens
D : aperture diameter ( supposed to be at the second lens location )
L : distance between lenses
Dr : maximum diameter of the image spot
dx1 : depth of field calculated as the algebric distance from the front focal point of the first lens to one of the limit positions
dx2 : depth of field calculated as the algebric distance from the front focal point of the first lens to the other limit position
dx :total depth of field
