The Canon EF 16-35mm f/2.8L II USM Autofocus Lens is a high performance, water-resistant, and ultra wide-angle L-series lens. The optical design has been improved to offer better edge-to-edge image quality. In addition, it features 3 high-precision aspherical lens elements, each of a different type: ground, replica and GMo. Improved AF algorithms will help the lens to focus faster and quieter.
All of the "L" lenses deliver optical performance equal to similar fixed focal lengths lenses, and the USM lenses do this with silent, high-speed focusing. Distortions and aberrations are remarkably small for a lens of such amazing capabilities.