Tokina gives full-frame photographers a 24mm wide-angle to 70mm moderate telephoto lens with a fast F/2.8 aperture at an affordable price. The new groundbreaking proprietary optical design uses three precision molded all-glass aspherical lens elements to achieve excellent contrast, sharpness, and corrects for spherical aberrations. One of these elements is very special and difficult to manufacture. G09, is made from Super-Low Dispersion (SD) glass and is a large sized aspherical element. These lens elements also yield even illumination across the whole image sensor. The Tokina 24-70MM lens also uses three SD (super-low Dispersion, FK01 and FK03) glass elements in the rear groups to control chromatic aberrations. All these specialized lens elements work in consort to yield maximum resolution to match the 50-megapixel DSLR cameras entering the market. The fast constant F/2.8 aperture makes viewing and auto focusing possible in very low light situations while keeping some flexibility for setting the shutterspeed. In addition to excellent optics, the lens incorporates a fast moving SDM (SilentDrive-Module) motor to greatly reduce AF noise making it one of Tokina's quickest and quietest auto-focusing lenses to date. Tokina's exclusive One-touch Focus Clutch Mechanism allows the photographer to switch between AF and MF simply by snapping the focus ring forward for AF and back toward the lens mount for manual focusing. The lens is also designed to stand up to the rigors of daily use by photographers in a wide variety of shooting conditions and environments.