Spanning a versatile wide-angle range the ef 17-40mm f 4l usm lens is a canon l-series zoom featuring a constant f 4 maximum aperture along with a sophisticated optical design and durable construction. One super ud element is featured along with three aspherical elements which help to reduce both chromatic and spherical aberrations in order to realize high sharpness clarity and color accuracy. A super spectra coating has also been applied to suppress flare and ghosting for improved contrast in difficult lighting conditions. Complementing the imaging capabilities this wide-angle zoom also features a ring-type usm and internal focusing mechanism to afford quick quiet and accurate autofocus performance as well as full-time manual focus override. Additionally this lens is weather-sealed for use in trying environmental conditions.
