The Canon EF-mount Tokina ATX-I 11-20mm f/2.8 CF is an ultra-wide zoom that is ideal for landscape, architecture, interior, and astrophotography applications. The optical design incorporates a series of aspherical and low dispersion elements, which suppress a variety of aberrations for improved sharpness and clarity. Ultra-wide zoom is designed for APS-C-format Canon EF-mount cameras and provides a 17.6-32mm equivalent focal length range. Its bright f/2.8 constant maximum aperture affords consistent brightness throughout the zoom range and benefits working in available lighting conditions. Three SD (super-low dispersion) elements control color fringing and chromatic aberrations throughout the zoom range for greater clarity and color accuracy.  Individual elements are also multi-coated to resist flare and ghosting for greater contrast and color accuracy when working in bright or backlit conditions.  One-Touch Focus Clutch Mechanism avails quick switching between autofocus and manual focus settings by either pushing or pulling the focus ring. When set to manual, focus response is especially smooth and benefitted by hard stops at the ends of the focusing range. It's Nine-blade diaphragm contributes to a smooth bokeh quality.