Only that x,y designate optional points in the X,Y axes , and z of equation is a distance of the optical axis direction from {HEAD ~ an optional point} of {TAIL ~ coaxially aspherical surface} to the XY plane 
of
Only that x , y designate optional points in the X , Y axes , and z of equation is a distance of the optical axis direction from { HEAD ~ an optional point } of { TAIL ~ coaxially aspherical surface } to the XY plane
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH