Method
The f/# and NA are calculated from the aperture size (or Gaussian 1/e² beam width) and the focal length
in air. Known ratios, together with the light wavelength, allow spot size calculations.
This calc is accurate to ~2 significant figures.
For a Gaussian beam (as opposed to a homogemeous beam), the "Aperture diameter"
is the 1/e² diameter, and a large lens size is assumed. For a truncated
beam, the appropriate spot size ratio should be applied (see the relevant calc,
e.g. 1.12 for truncation at the 1/e² diameter). The resolving power is considered to be 1/2 of the
Airy disk diameter, and the Rayleigh length is calculated here in air.
