Parameters
fi1: effective focal length of the first lens
fi2: effective focal length of the second lens
x: distance from the first lens to the object plane
xp: distance from the first lens to the aperture in the object space
D: aperture diameter in the object space
m: magnification
xi: distance from the second lens to the image plane
xpi: distance from the second lens to the aperture in the image space
NA: numerical aperture in the object space
NAi: numerical aperture in the image space
