We consider a new property of an optimal control problem called a lens. A lens is an interior point in the state-control phase plane where-given the value of the state variable-there is only one control value satisfying the necessary optimality conditions and-given the value of the control variable-there is only one state value satisfying the necessary optimality conditions. We build a simple model that generates a lens and give necessary and sufficient conditions under which a lens occurs. (C) 2007 Elsevier Ltd. All rights reserved.