The disclosure concerns an optical system of a microlithographic projection exposure apparatus. To permit comparatively flexible and fast influencing of intensity distribution and/or the polarization state, an optical system includes at least one layer system that is at least one-side bounded by a lens or a mirror. The layer system is an interference layer system of several layers and has at least one liquid or gaseous layer portion with a maximum thickness of one micrometer (μm), and a manipulator for manipulation of the thickness profile of the layer portion.