Details of an experimental demonstration of a contact hole imaging system are reported in which the depth of focus is increased by a factor of about 3.5 using annular illumination. Due to spatial filtering and nonlinearity of the photoresist, the resolving power was enhanced by 52% and it was possible to pattern a 0.28 um contact hole in photoresist deposited on a silica substrate. This technique is capable of fabricating sub-quarter micron holes using excimer laser radiation at 193 nm.