This article has described the Spot Diagram, FFT PSF, and Huygens PSF. In summary:

• Spot Diagrams show ray aberrations, but do not consider diffraction.
• The FFT PSF considers diffraction, is applicable to most optical systems, but makes a few assumptions.
• The Huygens PSF considers diffraction, is applicable to virtually all imaging systems, and makes fewer assumptions than the FFT PSF.
• The Huygens PSF may be used with non-sequential systems, even those that do not form images.

References

1. Goodman, Joseph W., Introduction to Fourier Optics, McGraw Hill
2. ZEMAX Optical Design Program User's Guide, ZEMAX Development Corporation