A projection objective for microlithography is used for imaging an object field in an object plane into an image field in an image plane. The projection objective comprises at least six mirrors of which at least one mirror has a freeform reflecting surface. The ratio between an overall length (T) of the projection objective and an object image shift (dOIS) can be smaller than 12. The image plane is the first field plane of the projection objective downstream of the object plane. The projection objective can have a plurality of mirrors, wherein the ratio between an overall length (T) and an object image shift (dOIS) is smaller than 2.