Provided is an infrared imaging optical system with an advantageously varying focal length across the field of view. More specifically, in a particular embodiment, provided is an inverse telephoto lens group having a field of view on the order of at least two radians. The focal length of the lens group is structured and arranged to vary in a pre-determined fashion across the field of view. An IR detector is optically coupled to the inverse telephoto lens group. A pupil is disposed between the IR detector and the inverse telephoto lens group.