While it cannot be claimed that this Quartet solution is a practical one, the problem did not require it to be so. It does, however, illustrate several important features of real lenses, and is shown to lie in performance between photographic and microlithographic lenses. The problem specifically excluded catadioptric designs, but a solution that ignores this rule is shown to give a significantly better minimum, illustrating that even the global optimum is local to the space defined by a given set of constraints.
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David M. Williamson, "Monochromatic quartet explained," Proc. SPIE 10263, Lens Design: A Critical Review, 1026306 (1 July 1992);