The EF 17-40mm f/4L USM, like all of the Canon "L" lenses, delivers optical performance equal to similar fixed focal length lenses, with silent, high-speed focusing. Distortions and aberrations are remarkably small for a lens of such amazing capability. It combines 1 high-precision molded glass aspherical lens and 2 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.
All of the focusing and zooming action is done internally, with no changes of the size of the lens. It only uses lead-free optics to help the environment, and features a circular diaphragm for improved background blur. The housing is water and dust resistant to help ensure reliable performance for both professionals and enthusiasts. Like many "L" lenses, it uses 77mm filters, but also has a rear gelatin filter slot behind the lens mount.
- Fixed f/4.0 Aperture Through Zoom Range
- UD Lenses Minimize Chromatic Aberrations
- Reduced Spherical Aberrations
- Highly Resistant to Dust and Water
- 7-Blade Diaphragm For Beautiful Bokeh
- Lens Coatings Minimizes Ghosting
- Fluorine Coating Reduces Fingerprints
|Sensor Size||Full Frame|