Projection objective and method for optimizing a system aperture stop of a projection objective

In certain aspects, the disclosure relates to a projection objective, in particular for a microlithography exposure apparatus, serving to project an image of an object field in an object plane onto an image field in an image plane. The projection objective includes a system aperture stop and refractive and/or reflective optical elements that are arranged relative to an optical system axis. The centroid of the image field is arranged at a lateral distance from the optical system axis). The system aperture stop has an inner aperture stop border which encloses an aperture stop opening and whose shape is defined by a border contour curve. The border contour curve runs at least in part outside of a plane that spreads orthogonally to the optical system axis.