From Manufacturer:
Covering a useful wide-angle focal length range, the EF 16-35mm f/4L IS USM is a Canon L-series lens distinguished by its constant f/4 maximum aperture, sophisticated optical design, and use of image stabilization to realize sharp imagery. Two ultra-low dispersion elements are used to greatly reduce color fringing and chromatic aberrations throughout the zoom range for high clarity and color accuracy, and a Super Spectra coating has also been applied to suppress flare and ghosting for improved contrast. Three aspherical elements are also featured and help to control distortion and minimize spherical aberrations for improved sharpness.

Complementing the imaging capabilities, this wide-angle zoom also features a four-stop-effective Image Stabilizer system that minimizes the appearance of camera shake for sharper handheld shooting. A ring-type USM and internal focusing mechanism also afford quick, quiet, and accurate autofocus performance, as well as full-time manual focus override. Additionally, this lens is weather-sealed and has a protective fluorine coating on the exposed elements for use in trying environmental conditions.