While an excellent ultra-wide zoom lens for film SLR cameras, the Canon EF 17-40mm f/4L USM Lens
is especially well suited for use with EOS digital SLR cameras, which
usually have an imaging sensor size smaller than the 35mm format. When
used with such digital SLR cameras, the 17-40mm focal length range
gives an angle of view approximately equivalent to that of a 27-64mm
lens upon 35mm cameras, making it an ideal medium-aperture standard
zoom lens.
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.
The 17-40's minimum focusing distance is just 11" (28cm) throughout its
entire range of focal lengths, which makes it very useful for close-up
work. 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.
� An autofocus super wide angle zoom lens for both film and digital cameras
� Produces 27-64mm equivalent focal length with "APS-C" size digital
EOS SLR cameras (such as the EOS 20D, EOS 30D, EOS Digital Rebel, EOS
50D, etc.)
� Uses 3 aspherical lens elements and Canon Super UD ultra-low dispersion glass
� Provides professional levels of moisture and dust resistance
� Internal zooming and focusing
� Circular aperture diaphragm for better, more natural looking background blurring
� Accepts 77mm filters, as well as gelatin filters upon the rear of lens
