The Canon EF 300mm f/4L IS USM AF Lens  is controlled by a 16-bit microcomputer that minimizes blurred images due to camera shake. A special mode intelligently compensates just for up & down movement, providing sharp, stable images for panning. The optical design that compensates for all types of aberrations from infinity to the closest shooting distance of 4.9'. Two ultra-low dispersion (UD) glass elements ensure clear images and accurate color. Image Stabilization technology gives you the equivalent effect of up to 2 full shutter speeds slower handholdability than a conventional lens. And Canon's advanced ultrasonic focusing motor (USM) and rear focusing design provide silent, high speed autofocusing with fulltime manual focus override.