The eigenfunctions of the integral equation describing the image-formation process are used for studying the degrees of freedom of coherent images from point-like-element pupils. The possibility of determining the number of effective degrees of freedom without solving the integral equation is demonstrated for an important class of cases. Finally, the number of effective degrees of freedom is explicitly calculated for cases of practical interest.
F. Gori and G. Guattari, "Degrees of freedom of images from point-like-element pupils," J. Opt. Soc. Am. 64, 453-458 (1974)