The interesting phenomenon of CD undersizing has been predicted by simulations to occur in high contrast/high exposure margin resists imaged with high numerical aperture steppers. Under these conditions the out-of-focus images of dense arrays, which are 1:1 at best focus, print too small. The phenomenon is particularly prevalent at geometries which are larger than those near the resolution limit of the imaging tool, and is most apparent when the resist surface is moved away from the focal plane. In this study CD undersizing is demonstrated experimentally and explored in more detail using computer simulation for both actual state-of- the-art i-line resist/imaging systems, and for hypothetical resist systems having monotonically increasing contrast. The origin of the effect is due to the characteristics of the aerial image intensity profile of partially coherent illumination. In particular, the relative intensity at the mask edge may increase with defocus depending upon the spatial frequency (linewidth), partial coherence, and numerical aperture of the imaging system. This results in an `over-exposure' condition and hence undersizing of the out-of-focus images may occur. The same aerial image characteristics give rise to the familiar `pipe-bowl' effect observed in the (zero defocus) mask- linearity plots, which are frequently used to characterize positive resists. The magnitude of undersizing decreases with increase in partial coherence, increases with resist contrast and may lead to retrograde profiles.