Tokina AT-X 16-28mm f/2.8 Pro FX Lens for Canon

NEW Silent DC Motor with GMR senor 
The 16-28 f/2.8 uses a newly developed silent DC motor that allows the lens to focus faster and more quietly then previous generations. The DC motor coupled with a new GMR magnetic AF sensor work together to increase AF Speed. 

Ashperical and Super-low Dispersion glass elements 
A new, 56mm in diameter, large sizes aspherical glass element is incorporated into the front lens group, while there are 2 more aspherical elements in the rear group. 3 SD super-low dispersion glass elements are also incorporated through-out the optical design to reduce chromatic aberration, give maximum resolution, more even brightness and distortion correction. 

One-Touch Focus Clutch 
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 camera to focus 
manually. There is no need to change the AF/MF switch on Nikon camera bodies* and there is no second AF/MF switch on the lens for Canon, everything is accomplished by the focus ring.