In the case of a lens system (1), in particular in the case of a projection lens system for semiconductor lithography, it is possible to exchange at least one optical element, in particular the final optical element in the beam direction in the form of an end plate (3). For adjusting the exchangeable optical element (3), an optical quality fit with a fitting surface (6) is set between the optical element (3) and a mount (5) or a part connected to a mount (5).