An imaging optical system has an optical axis and a refractive optical group including a lens lying on the optical axis. At least one lens of the refractive optical group lying on the optical axis has an aspheric front surface and an aspheric back surface. An image surface lies on the optical axis, and the refractive optical group forms an image at the image surface. The image surface is preferably substantially planar. A detector, such as a focal plane array detector, lies on the optical axis such that a ray of light passing through the lens is incident upon the detector. It is preferred that there are two lenses lying on the optical axis, with each lens having an aspheric front surface and an aspheric back surface. The imaging optical system is suitable for use as a large-aperture, wide-field-of-view, compact system having an F number of less than about F/1.5, a field of view of more than about 45 degrees, and a telephoto ratio of less than about 2.0.