This invention involves an optical system in which optical radiation containing a wavelength .lambda. is directed onto a patterned mask, in order to form an image feature on a photoresist layer located on the image plane of the system. The patterned mask has a main object feature, which has the form of the image feature. The object feature has a portion whose width is everywhere less than (1.5).lambda./NA, where NA is the numerical aperture of the image side of the system. An assist feature whose width is everywhere less than (0.5).lambda./NA is located on the mask in a neighborhood of the portion of the main object feature. Advantageously, the optical radiation is directed through an annular aperture ("off-axis illumination") in an opaque screen and through a collimating lens onto the mask. In one exemplary situation, the assist feature is located outside the main object feature and has a distance of closest approach to the main object feature that is everywhere equal to less than .lambda./NA. In another exemplary situation, the assist feature is located inside the main object feature. In addition, advantageously the object and the assist features in the mask are defined by localized clear regions (optical transmission coefficient is equal to approximately unity) located in a relatively opaque field (optical transmission coefficient T=approximately 0.10), or are defined by localized relatively opaque regions located in a clear field, as the case may be.