An infrared continuous zoom telescope or afocal lens system using a diffractive surface for color correction. The lens system comprises an objective lens, an eyepiece lens, a plurality of lenses disposed between the objective lens and the eyepiece lens to provide the zoom function and a diffractive pattern disposed at a surface, preferably the interior surface and preferably integral therewith, of the objective lens to provide color correction at a predetermined wavelength of light passing through the objective lens. All of the lenses are preferably germanium. The diffractive pattern comprises a plurality of spaced ring-shaped steps, each ring concentric with the center of the objective lens and each step having a height equal to the predetermined wavelength for which the lens system is designed. The plurality of ring-shaped steps are formed in accordance with a known formula.