The PSF of an optical system is the irradiance distribution that results from a single point source in object space. A telescope forming an image of a distant star is a good example: the star is so far away that for all practical purposes it can be considered a point.

Although the source may be a point, the image is not. There are two main reasons. First, aberrations in the optical system will spread the image over a finite area. Second, diffraction effects will also spread the image, even in a system that has no aberrations.

ZEMAX has three basic types of PSF calculations: a geometric (no diffraction) spot diagram, and the diffraction based FFT and Huygens PSF. This article will discuss the basic theory and provide some guidance as to the proper use of each type of PSF.