Parameters
x: distance from the vertex of the first surface to the object
y: height of the object
ns: refraction index of the lens for the smallest wavelength
nl: refraction index of the lens for the the largest wavelength
R1: radius of curvature of the first surface
R2: radius of curvature of the second surface
e: center thickness
xir: distance from the vertex of the second surface to the image for the largest wavelength
xib: distance from the vertex of the second surface to the image for the smallest wavelength
dxi: axial chromatism of the lens
yir: height of the image for the largest wavelength
yib: height of the image for the smallest wavelength
dyi: height chromatism of the lens
CrT: ratio dyi/ yi with yi = (yir + yib)/2
