Di: Digitally Integrated Design, is a designation Tamron puts on lenses featuring optical systems designed to meet the performance characteristics of digital SLR cameras. This lens employs an IF (Internal Focusing) system that maintains the overall length of the lens during focusing. Uses two LD (Low Dispersion) glass elements that effectively compensate for chromatic aberration. Provides high-contrast, outstanding image quality and a soft out-of-focus background effect. Incorporates an AF/MF one touch switchover mechanism built into the focus ring(for Canon & Nikon only). Super performance series provides high performance through outstanding design specifications. FEC (Filter Effect Control): Features the new FEC (Filter Effect Control) function that enhances operational ease of Polarizing Filter use. The new Filter Effect Control is designed to rotate the filter to the desired position while the hood is attached. This is accomplished by turning the FEC ring that in turn rotates the portion of the filter mount where the filter is mounted.