1. A method of determining a point spread function at a specified point in an optical image, the method comprising: with computing equipment, generating a data structure associating predetermined point spread functions with predetermined points in the optical image and with wavelengths; with the computing equipment, determining the point spread function at the specified point according to the predetermined point spread functions, wherein the point spread function associated with the specified point is not stored in the data structure; and with the computing equipment, determining the point spread function based on a specified wavelength by interpolating a difference between the specified wavelength and a wavelength associated with at least one of the predetermined point spread functions, wherein the point spread function associated with the specified wavelength is not stored in the data structure.