Images from high aspect ratio annulus apertures are spatially filtered in a coherent optical processor for equalization of the modulation transfer function. The imagery is relatively insensitive to spherical aberration, field curvature, and longitudinal color; and the depth of focus is substantially greater than that of conventional imagery with comparable resolution.
J. T. McCrickerd, "Coherent Processing and Depth of Focus of Annular Aperture Imagery," Appl. Opt. 10, 2226-2230 (1971)