An optical system for projection photolithography is disclosed. The optical system is a modified Dyson system capable of imaging a large field over both a narrow and a broad spectral range. The optical system includes a positive lens group having a positive subgroup of elements that includes at least a plano-convex element and a negative subgroup that includes at least a negative meniscus element. The lens subgroups are separated by a small air space. The positive and negative subgroups constitute a main lens group arranged adjacent to but spaced apart from a concave mirror along the mirror axis. The system also includes a variable aperture stop so that the system has a variable NA. A projection photolithography system that employs the optical system is also disclosed.