where NA is the numerical aperture of the lens, NA = n x sinθ, where n is the refractive index of the medium between the projection lens and the wafer, through which the exposure light passes.
Aperture diaphragm An adjustable diaphragm located in the illumination optics, which controls the numerical aperture of the illuminating beam and affects the brightness of the beam.
where N is the uncorrected f-number, "NA" is the numerical aperture of the lens, and m is the lens's magnification for an object a particular distance away.
as stars are point sources, the numerical aperture of the lens is not the determining factor in the exposure of stars.
The actual _area_ of the lens is the determining factor.
Thus a 200/4 has as much light-gathering area as a 50mm f1.0 ! ...
The term used to universally describe the lens opening is the "numerical aperture" (inverse of twice the f-stop).
 See also: Focal, Meter, Aperture, Lens, Light
  
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