A projection lens for a EUV microlithographic projection exposure apparatus comprises a diaphragm (BL) which is arranged at a distance (D) in front of a mirror (S2) of the lens. The diaphragm (BL) has a non-round aperture with an edge contour that may be configured such two rays of a light bundle disposed symmetrically with respect to a chief ray are treated equally, i.e. either both rays pass through the diaphragm aperture or both are blocked by the diaphragm.