A very high-aperture, purely refractive projection objective having a multiplicity of optical elements has a system diaphragm (5) arranged at a spacing in front of the image plane. The optical element next to the image plane (3) of the projection objective is a planoconvex lens (34) having a substantially spherical entrance surface and a substantially flat exit surface. The planoconvex lens has a diameter that is at least 50% of the diaphragm diameter of the system diaphragm (5). It is preferred to arrange only positive lenses (32, 33, 34) between the system diaphragm (5) and image plane (3). The optical system permits imaging in the case of very high apertures of NA.gtoreq.0.85, if appropriate of NA.gtoreq.1.