In this article, the imaging principles of projection steppers for optical lithography, using standard transmission masks, are reviewed, and compared with the image formation using various types of phase shifting masks (PSM), in order to get a better insight in the working principles of the various PSM techniques. The printability of periodic lines using a standard transmission mask is compared with the use of an alternating shifter PSM and the dependence of imaging on spatial coherence is discussed extensively. Furthermore the imaging of isolated lines is discussed, using a bright field standard transmission mask, a chromeless phase edge PSM, and a dark field rim PSM. The coherence dependence of an alternated shifter is verified experimentally. Experiments also confirm the superior performance of a chromeless phase edge PSM to print isolated lines.