An objective and a method for operating an objective, in particular a projection objective or an illumination objective for microlithography for imaging a reticle onto a wafer, with a plurality of optical elements that are arranged along a ray path, wherein at least one optical element of a first kind (1) is provided, which is irradiated only partially by a ray bundle, wherein the one or more optical element(s) of the first kind are rotatably mounted or positionable about the optical axis or an axis parallel thereto, wherein, for each optical element of the first kind at least two mounting positions are provided, and wherein the rotation angle between the two mounting positions is defined by the surface (7) irradiated by the ray bundle such that, in the various mounting positions, the surfaces irradiated by the ray path do not overlap.