This ground-breaking high-speed mid-range zoom is prized by pros and serious shooters for its fast F/2.8 constant aperture, evenness of illumination, and its outstanding imaging performance, and by all photographers for its compact size and reasonable weight that make it feel like an ordinary standard zoom. These admirable characteristics have been achieved by the use of special XR and LD glass, the efficient use of aspherical elements, and non-rotating internal-focus (IF) design.