Optical elements shaped in accordance with preferred analytic functions which permit the elements to be relatively rotated about one or more pivots decentered with respect to an optical axis to simulate the dioptric action of a well corrected rotational lens element of variable power that can be used to maintain focal setting over a large range of object distances. The optical elements can be used in pairs, singly with mirror images of themselves, or they can be incorporated in more elaborate systems to provide focusing action.