1. An optical system comprising: three or more lens elements made of two or more different optical materials, wherein none of the optical materials has an internal transmission less than 10% at a wavelength of 310 nm when measured at a thickness of 10 mm, and wherein all lens elements are made from optical materials having an Abbe number higher than 40; and wherein the optical system is well-corrected over a wavelength band that ranges from about 315 nm to about 1100 nm.