The area of an image plane and the number of discrete elements of area sampled in image plane scanning impose rigid requirements for suitable optical systems. At sufficiently long wavelengths and for sufficiently large numbers of elements, each of small size, the required high quality, wide angle, and small F/number optical systems are impracticable. When the optical system is designed with maximum allowed F/number, a required angular resolution fixes the light gathering power of the system. The availability of better optical materials and development of suitable optical systems only extends the wavelength or element density where a transition to impracticable optical systems occurs.
Charles S. Williams, "Limitations on Optical Systems for Images of Many Discrete Elements of Area," Appl. Opt. 6, 1383-1385 (1967)