This new lens does what many pros thought couldn't be done - replace the previous L-series 28-70 f/2.8 lens with something even better. Extended coverage to an ultra-wide angle 24mm makes it ideal for digital as well as film shooters, and the optics are even better than before with two Aspherical elements and a totally new UD glass element. It's now sealed and gasketed against dust and moisture, and a new processing unit makes the AF faster than ever.In order to achieve critical autofocus, the elements within a camera lens have to move quickly, quietly, and with exacting precision. To accomplish this, Canon developed the world's first lens-based Ultrasonic Motor (USM). Based on a totally new technology, the motor spins by ultrasonic oscillation energy. In effect, instead of a large, noisy drive-train system, electronic vibrations created by a piezoelectric ceramic element power the mechanical action of the lens. In a very broad comparison, it's like the difference between a mechanical watch with gears and springs and quartz-timed watch movement. Therefore, torque is constant and start and stop functions virtually instantaneous.By making the autofocus operation almost inaudible and by accelerating the autofocus speed, this type of motor greatly expands the shooting possibilities for Canon photographers.