ABSTRACT
The distribution of the density of rays in an image is the geometrical point-spread function (PSF) of an optical system for the point object under consideration. The geometrical PSFs for primary aberrations is compared with the corresponding diffraction PSFs. This comparison demonstrates the diffractions PSFs undergo dramatic changes as the aberration increases; the geometrical PSFs simply increase linearly in size. For optical design, it is reasonable to use the size of the spot diagrams as a qualitative measure of lens design quality until it becomes smaller than the Airy disc. For quantitative assessment of the image, diffraction PSFs must be examined.
