OpticStudio provides two methods to compute PSF. One is the FFT PSF. This is where OpticStudio takes the wavefront at the exit pupil and Fourier Transform it to get the PSF. The other is the Huygens PSF. The Huygens PSF uses a very different approach. It samples the wavefront at the exit pupil. Each pupil sampling point then becomes a Huygens wavelet. OpticStudio propagates all these Huygens wavelets individually, either as plane waves or as spherical waves, from exit pupil to the Image surface, and then coherently sums all these Huygens wavelets to get the final PSF irradiance distribution on the Image surface. Huygens PSF is more computationally intensive, but it is also more robust and makes less assumptions. In many systems, the FFT PSF and Huygens PSF may provide similar results, however, there are also cases where they will not agree, simply because the FFT PSF makes two important assumptions: 1. The image surface lies in the far field of the optical beam. This means the computed PSF is only accurate if the image surface is fairly close to the geometric focus for all rays; 2. The chief ray does not form a large angle with the image surface because the FFT PSF computes the PSF on a plane normal to the chief ray. Therefore, if you have a system that has a large amount of defocus, or if you launch rays from a field point where the chief ray forms a large angle (>20 degrees) with the image surface, you should trust Huygens PSF method more. In general, we recommend that when the two methods give different results, the Huygens method is more reliable because it does not make similar assumptions as the FFT PSF.   

The Help file provides a detailed description on these two PSF computations at

The Analyze Tab (sequential ui mode) > Image Quality Group > PSF > FFT PSF

The Analyze Tab (sequential ui mode) > Image Quality Group > PSF > Huygens PSF