The Mask Error Enhancement Function (MEEF) serves to amplify reticle errors. This can lead to proximity effects and bias problems that are much larger than would be expected from the normal reduction factor of the imaging system. The economic impact on reticle specifications can be severe. This paper examines the theoretical description of the MEEF for dark features: isolated lines, isolated posts, and dense 1:1 line/space features. MEEF for dense features is found in general to be smaller than 1 over a wide range, while MEEF for isolated features is always greater than 1. This 'MEEF Gap' between isolated and dense features may help to explain the sensitivity of OPC to isolated and dense bias.