Looking to take things up a notch, Canon's EF 16-35mm f/2.8L III USM Lens embraces a revamped optical design with modern coating technologies that promise improved outer and corner sharpness along with minimal distortion and enhanced peripheral illumination. A member of Canon's legendary L series, this full-frame 16-35mm delivers outstanding image quality and a professional-grade durability. It also offers a fast and constant f/2.8 maximum aperture for consistent performance and light transmission throughout the zoom range.
With the incorporation of two large diameter GMO dual surface aspherical elements and one ground aspherical element, the lens will minimize distortions throughout the zoom range as well as maintain edge-to-edge sharpness and illumination. Along with those elements, the lens utilizes two Ultra-low Dispersion glass elements which minimize chromatic aberrations. Also, it uses both a Subwavelength Coating and an Air Sphere Coating to minimize flare and ghosting.
In terms of handling, the 16-35mm uses a ring-type USM focusing motor combined with a high-speed CPU and optimized algorithms for lightning fast and accurate autofocus. The lens has an internal focus design and offers full-time manual focus override so users can make tweaks using the physical focusing ring regardless of mode. Along with all of this, the aperture diaphragm uses nine rounded blades to create smooth bokeh.
For professional use, the lens is dust- and water-resistant, using seals throughout to prevent intrusion of particles, especially when shooting in inclement weather. Additionally, it uses a fluorine coating on the exposed front and rear elements to reduce smears and make cleaning easier.
 
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https://www.bhphotovideo.com/c/product/1274708-REG/canon_ef_16_35mm_f_2_8l_iii.html