The Canon EF 17-40mm f/4L USM lens is a member of a suite of Canon zoom lenses that retain a constant f4 maximum aperture throughout the zoom range. The Canon EF 17-40mm f/4L USM  also proves useful for shooting in cramped interiors.

The Canon EF 17-40mm lens focuses as close as 0.28m. And Canon EF 17-40mm offers both Canon full-time manual focus and a powerful ring-type USM for fast and silent AF.

This Canon EF 17-40mm lens is especially well suited for digital SLR cameras which usually have an imaging sensor size smaller than the 35mm format.

The Canon EF 17-40mm f/4L USM Lens has a constant f/4 maximum aperture, and offers the choice of screw-in 77mm filters or a holder in the rear of the lens for up to three gel filters. Finally, Canon EF 17-40mm f/4L USM Lens offers weather-resistant construction similar to other high-end L-series lenses.

The 17-40mm f/4L USM len contains 12 glass elements, arranged in three groups, with three aspherical elements to counteract the aberrations that beset wide-angle zooms.

Different types of aspherical construction are involved and the lens includes two ultra-low-dispersion (UD) elements to minimise chromatic aberration. The optics of the Canon EF 17-40mm f/4L USM lens are contained within a black body that is built for dust and moisture resistance, and the front filter mount remains fixed in position while the lens is focused, making it ideal for use with angle-sensitive filters, such as polarisers and graduates.

Focal length settings are engraved on the lens barrel, a useful feature when you want to set precise magnifications. In addition, the lens includes a depth-of-field gauge, which many photographers appreciate. The Canon EF 17-40mm f/4L USM Lens lens is supplied with a shallow 'petal-shaped' EW-83E lens hood, 77mm lens cap and rear dust cap plus an LP1319 soft case.

The EF 17-40mm f/4L USM Lens Features:

• Ultra-wide high performance zoom lens offering excellent image quality
• Dust-proof and moisture-proof construction
  Rear focusing and ring USM for silence and high-speed AF
• 2 UD and 3 types of aspherical elements reducing chromatic aberration of magnification and providing superb image quality
• Non-rotating front element during focus

Specifications:

Focal Length & Max. Aperture: 17-40mm f/4.0
Filter Diameter: 77mm
Magnification with Extension Tube EF12: 0.83x - 0.32x
Magnification with Extension Tube EF25: 1.02x - 0.70x
Diagonal Angle of View: 104 - 57 (43.2mm)
Horizontal Angle of View: 93 - 49° (36mm)
Lens construction: 12 elements in 9 groups
Minimum aperture: f/22
No. of aperture blades: 7 (circular aperture)
Closest focusing distance (m): 0.28m
Focus drive: Ultrasonic
Dimensions: 83.5mm dia. x 96.8mm
Weight: 475g

See the Canon EF 17-40mm f/4L and full range