The super-wide angle's useful range of view can be conveniently utilized by simply rotating the smooth zoom ring.SLD (Special Low Dispersion) Glass is used for effective compensation of color aberration. Two pieces of molded glass aspherical lens elements offer excellent correction for distortion as well as all types of aberration. It is an ideal lens for Digital SLR Cameras as well as film SLR cameras.HSM (Hyper Sonic Motor) provides quiet, high-speed autofocus shooting, as well as full-time manual focus. It is easy to revert to Manual Focus by simply turning the focusing ring and overriding the AF.An inner focus system provides high performance and convenience. The non-rotating lens barrel perfectly suits the matched petal shaped lens hood. more info