This is a high-magnification standard zoom lens covering wide-angle to medium-telephoto range. This is the first Canon zoom lens that uses rear focusing system. Lenses which carry out focusing by moving one or more lens groups behind the diaphragm position are called rear focusing lenses. The rear focus technology for zoom lenses was established during development of this lens and it contributed to the development of following multi-lens-group EF zoom lenses, especially telephoto zoom lenses. A molded glass (GMo) aspherical lens element (12th) is used to correct astigmatism to achieve sharp definition and to make the lens compact. The AF system is driven by a ring USM (Ultrasonic Motor), enabling full-time manual focusing as well.