This paper estimates how the degree of divergence/convergence of a beam of monochromatic light that illuminates an aperture affects the geometrical size and spatial coordinates of the axial energy spectra in terms of a quasi-geometrical approach. Analytical expressions are obtained that make it possible to allow for and minimize the errors of formation of near-field diffraction images.
V. N. Il'in and A. V. Dubeshko, "Estimating how the nonparallelness of an object-probing beam affects the parameters of the axial diffraction extrema," J. Opt. Technol. 70, 544-547 (2003)