An optical system, for example a lens for a photolithography tool, includes a group of optical elements (L1, L2) that each comprise a birefringent cubic crystal such as CaF2. The crystal lattices of the crystals have different orientations, e.g. for reducing the overall retardance of the group by mutual compensation. The [110] crystal axis of at least one optical element (L1, L2) is tilted with respect to an optical axis (34) of the system (10) by a predefined tilting angle (θ1, θ2) having an absolute value between 1° and 20°. This reduces the magnitude, but not significantly changes the orientation of intrinsic birefringence. By selecting an appropriate tilting angle it is possible to achieve a better performance of the optical system. For example, the overall retardance of the optical system may be reduced, or the angular retardance distribution may be symmetrized.