One form of optical proximity effect that further complicates lithography is the unexpected response of the printed image to small perturbations [critical dimension (CD) errors] in the reticle. In this way mask CD errors are actually magnified (they are reduced by less than the reduction factor of the optics) during the optical transfer to the wafer. This effect will require even tighter specifications for mask CD control when the error magnification factor is significantly above unity. The effect is particularly pronounced for tight pitches of small features, but can also impact the printing of small isolated lines. Both resist and optical nonideal responses are involved in this mask error factor (MEF). This article discussed the optical effects that produce the MEF. This article will show where the MEF due to optical effects can be ignored and where they cannot when using 248 nm lithography with a high numerical aperture (NA) tool. We will demonstrate how the NA, partial coherence, and variations in focus can effect the mask error magnification factor. We will also show that resolution enhancement techniques can be used to reduce the mask error magnification factor. In particular, Levenson phase shift masks show particularly low mask error magnification factors for small lines. Far some applications it should be possible to design the mask so that the mask error magnification factor of the smallest features is significantly below unity. This would allow loosened reticle CD specifications and/or better CD control of the lithography process.