The EF 17-40mm f/4L USM Lens from Canon is a high quality "L" series wide angle zoom lens for full frame and APS-C size DSLR cameras. It combines one high-precision molded glass aspherical element and two replica aspherical lenses to achieve the extended zoom range, while also correcting for common aberrations that arise during zooming. A Super UD (Ultra-low Dispersion) glass element minimizes the chromatic aberration of magnification which tends to occur with wide-angle lenses, to help eliminate color bleed along the edges of the subject and achieve high image contrast and resolution.