A monocentric element, that is, one whose surfaces are concentric, can be valuable in an optical design. Such elements are, however, very difficult to fabricate. A departure from exact monocentricity can make the fabrication problem more tractable and can usually be achieved without loss of design performance. A relationship is derived that indicates the minimum departure from monocentricity necessary to make fabrication feasible.
Warren J. Smith,
"The Problem Of The Concentric Meniscus Element: A Possible Solution to the Lens Diesigner's Dilemma," Optical Engineering 27(12), 121039 (1 December 1988). http://dx.doi.org/10.1117/12.7978673