Consider a plano convex lens used to focus a collimated gaussian beam , if the beam diameter is almost the same as the lens diameter (45.7mm) we get a geometrical rms spot size of 51um 

if the entrance gaussian beam diameter is reduced to 20mm, we get a rms spot size of 4um because of reduced spherical Aberration : 

but if the entrance collimated beam diameter is 150mm , which is 3 times larger than lens diameter we get : 

Zemax reports a focus spot size of 2.2mm . Clearly this is not correct. 

How do we model the propagation of Gaussian beam limited by an aperture. How would the diffraction generated by the circular aperture affect spot size? would the effects be negligible or would the spot size degrade of a truncated beam? 

[Mod note: moved to more appropriate forum for OS-related discussions.]