Mask Error Enhancement Factor (MEEF) or commonly Mask Error Factor (MEF) has recently become an important metric in determining process requirements in the SIA roadmap. MEF, in general, varies inversely with CD and is often significantly above unity indicating that mask CD errors are in effect magnified during the optical transfer to the wafer. Until recently, the SIA roadmap indicated mask makers needed to allow in the reticle budget for a 1.5X MEF allocation. Discussion at recent industry workshops has indicated that this allocation may be underestimated. We have generated experimental results for vertical, horizontal, dense, and isolated lines as well as contact holes for feature sizes in the range of 150 - 250nm. MEF dependence on the lens, and its variation will be compared across several scanning exposure systems. Role of Numerical Aperture (NA) and role of different illumination settings including conventional, annular, and quadrupole will be measured and compared to the simple theoretical expectations. Finally, MEF will be studied across lens location and correlated to the aberrations. A significant difference in mask error factor between horizontal and vertical lines will also be described in terms of feature size and lens aberrations.