Question as described in the title. The spot size i mean here is neither RMS or GEO spot size, it is the calculation size like Airy Radius with diffraction effect consideration. Airy spot size is the min spot size in focusing distance, but how does OpticStudio calcuate the spot size in de-focus distance? what is (are) the formula(s)? Similarly for MTF calculation, just using a paraxial lens with known EFL and imaging F/#.
The Airy radius is just 1.22*primary wavelength/f#, and it does not depend on focal distance. RMS and Geo radii do depend on defocus, as they are calculated from ray landing coordinates on the surface, but Airy disk, EFL, F/# etc don't. Does that help?