As the critical dimension is reduced, the most severe Optical Proximity Effect (OPE) are the Iso/Dense Bias (IDB) and the Line-End Shortening (LES). Before using an automatic software to correct such effects, it can be interesting to find the Numerical Aperture (NA), Filling Factor ((sigma) ) couple which gives the best result in term of reduction of IDB and LES. This study focuses on the behavior of LES and IDB as a function of NA and (sigma) on 0.35 micrometer/I-line and 0.25 micrometer/DUV design rules. On both IDB, and LES, interesting results have been obtained. Results obtained for IDB confirm previously published data and show that it is possible to reduce, in a significant manor, the IDB in conventional illumination mode, by taking a suitable NA, (sigma) couple. Moreover we emphasize in this paper that (sigma) has no significant effect on LES unlike previously published data, and that it is possible to reduce LES to an acceptable level by tuning NA. Regarding the results obtained in this study, it appears to be possible to reduce both IDB and LES by fine tuning NA and (sigma) . Effectiveness of serifs and hammerheads in reducing LES is also discussed.