Doesn t ZEMAX already consider diffraction?
ZEMAX does support some diffraction calculations, such as the Diffraction PSF and MTF. However, these calculations are based upon geometric ray tracing. Rays are used to propagate through the entire optical system, and the path length of the rays is used to reconstruct the wavefront in image space. A single Fraunhofer diffraction step is then used to compute the PSF or MTF. When using the geometric optics model, all of the diffraction is assumed to occur in just the last propagation, from the exit pupil to the image. Diffraction that occurs at the lens apertures, and as the beam propagates between the lenses, is ignored. For many optical systems, including most imaging lenses, this simplified model is adequate. For other systems, it is not.
