 f/#, NA, and spot size

 
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.  