An illumination system of a microlithographic projection exposure apparatus comprises at least one transmission filter (66; 166; 266; 366; 466; 566; 666; 666'; 766, 766'; 866, 866') which has a different transmittance at least at two positions and which is arranged between a pupil plane (42, 60) and a field plane (52, 58). The transmittance distribution is determined such that it has field dependent correcting effects on the ellipticity. In some embodiments the telecentricity and/or the irradiance uniformity is not affected by this correction.