Canon EF-S 17-55/2.8 IS USM
The Canon EF-S 17-55/2.8 IS USM is just about the ultimate "general purpose" lens for Canon's line of APS-C DSLRs. Equivalent in full frame terms to a 27-88/2.8 lens it's suitable for everything from landscapes to portraits and the fact that it is f2.8 and has 3 stops of image stabilization makes it a very good lens indeed for hand-held low light work such as shooting interiors.
At a price of around $1080, this is the most expensive EF-S lens so far and may indicate a strong commitment on the part of Canon to support the EF-S mount for its prosumer DSLRs. Though it's not designated as an "L" series lens (it seems to be policy on Canon's part to restrict the "L" designation to full frame lenses), it appears to be very close in performance to such lenses, using two ultra-low dispersion and two aspheric elements to achieve maximum image quality.
21 February, 2006: Canon today strengthens its EF-S lens series with the Image Stabilized wide angle EF-S 17-55mm f/2.8 IS USM. With image quality and construction approaching that of Canon's renowned professional lenses, the EF-S 17-55mm f/2.8 IS USM becomes the fastest standard focal length zoom for EFS mount cameras such as the new EOS 30D. Combining constant f/2.8 aperture throughout the zoom range with Canon's latest 3-stop Image Stabilizer, it delivers outstanding wide angle performance and framing flexibility in low light conditions. Specialist optics such as UD glass and aspherical elements help deliver exceptional clarity, sharpness, contrast and colour accuracy.
The lens fits exclusively on those EOS cameras with an EFS lens mount the new EOS 30D, EOS 350D, and earlier EOS 20D and 300D models. (see appendix Technologies Explained for further information on the EFS mount). The 1.6x multiplication effect of these cameras' APS-C size sensor gives the lens an effective focal length of approximately 27-88mm.
To improve image clarity over the total image area, two Ultra-Low Dispersion glass (UD) elements are employed to eliminate secondary chromatic aberrations that otherwise reduce sharpness and introduce colour fringing. To ensure crisp, high contrast from one corner of the frame to the other, two aspherical lens elements have been introduced to avoid spherical aberrations that can soften images, especially at wider apertures. In combination with digital EOS CMOS sensor and DIGIC II image processor technologies, the EF-S 17-55mm f/2.8 IS USM delivers image quality surpassing that required for many commercial applications.
Fixed f/2.8 from 17 - 55mm
Superb optical quality with no change in aperture over the full focal length range allows photographers to set exposure at the widest aperture and concentrate on framing, without concern for adjusting shutter speed at longer focal lengths. The lens employs an almost perfectly circular barrel aperture, helping create attractive, even background blur.
The lens features Canon's latest 3-stop Image Stabilizer (IS) for dramatically reduced image blur caused by camera shake. IS significantly extends scope for shooting in low light conditions without flash, or for gaining depth of field. The 3-stop advantage allows a photographer shooting in low light to use a shutter speed of just 1/15 sec instead of 1/125, with no perceptible increase in image blur.
IS automatically detects when the camera is mounted on a tripod, thus preventing feedback loops between the IS sensor and stabilizer motor vibrations.
(See more information on Image Stabilizer, see appendix Technologies Explained').
Fast, silent auto-focus
The lens' USM motor employs ultrasonic frequency vibrations to drive auto focus with speed and near-silent operation. The constant torque motor moves the focus group at high speed, stopping with precision, accuracy and without overshoot the instant the lens arrives at the correct focus point. Photographers can switch to manual focus instantly and at any time simply by moving the manual focus ring without first having to switch out of auto focus mode.
Harmful reflections eliminated
By optimising Super Spectra lens coatings and lens element shaping, Canon's engineers have been effective in suppressing flare and ghosting more prone to occur with digital cameras due to reflection off the image sensor. By increasing light absorption, coatings reduce reflections off lens element surfaces to deliver crisp, undistorted images with natural colour balance.
EOS system integration
When used in conjunction with Canon's range of EX Speedlite flash units and the internal flash of the EOS 350D, EOS 20D and new EOS 30D, the lens passes distance information to the E-TTL II flash algorithm to improve flash exposure metering.
An optional lens hood, made exclusively for the EF-S 17-55mm f/2.8 IS USM, and pouch are available.
In keeping with Canon's Kyosei philosophy of living in harmony with the environment, the lens features only lead-free glass.
EF-S lens mount
The compact APS-C size sensor of select EOS cameras such as the EOS 30D, EOS 20D, EOS 350D and EOS 300D has paved the way for a range of relatively compact and lightweight EF-S mount lenses. These lenses project a smaller image circle than would otherwise be necessary to cover a full frame sensor or a frame of 35mm film. The S' in the EF-S nomenclature stands for the term short back focus'. This describes the system that allows for both a smaller image circle and positioning of the rear lens element closer than normal to the camera's sensor.
Cameras with an EF-S lens mount can accept both EF lenses and EF-S lenses, while EF-S lenses can only fit to cameras with an EF-S lens mount. A white lens mount index differentiates the lens from other EF lenses, which retain their traditional red marking. A rear rubber ring prevents any damage in the event that an inadvertent attempt is made to mount an EF-S lens to a camera with an EF mount.
IS works by using gyro sensors and circuitry to detect and measure unwanted vibrations. Tiny servo motors then create corresponding movement of a correcting lens group perpendicular to the optical axis. This alters the light path, returning the image to its correct position on the sensor or film plane. Unwanted movement is corrected fast enough to suppress effectively the vibrations of moving vehicles and aircraft such as helicopters.
1. Only compatible at tele.
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