An optical system, such as an illumination device or a projection objective of a microlithographic projection exposure apparatus, is disclosed. The optical system can include a polarization compensator which has at least one polarization-modifying partial element. The optical system can also include a manipulator by which the position of the at least one partial element can be altered. At least one operating mode of the optical system can be set in which the intensity, over a region which belongs to a plane perpendicular to the optical axis and which can be illuminated with light from the light source, does not exceed 20% of the maximum intensity in the plane, and the manipulator is arranged in the region.