It incorporates hybrid aspherical lens in the front lens group to minimize distortion and astigmatism, and molded glass aspherical lens in the rear lens group to minimize spherical aberration. (Sigma Corporation of America)

Now found in almost every EF lens, aspherical lens elements are particularly useful for large-aperture lenses, wide-angle lenses and high quality compact zoom lenses. (canon.co.uk)

It incorporates a floating focus system and uses two aspherical lens elements for minimizing distortion, spherical aberration and astigmatism. (SIGMA CORPORATION)