The digital SLR crop factor, or lens multiplication factor, has the effect of cropping the edges of an image and enlarging the central area. The end result is similar to using a longer lens, or a tele converter.
The cropping effect occurs because the sensor on most digital SLR cameras is smaller than a 35mm slide or negative. A "frame" of 35mm film is 36 x 24mm in size, while the digital sensor on Canon Digital Rebels (300D to 500D) and the EOS **D range (20D to 50D, and 7D) is only 22.5 x 15mm in size. A smaller sensor to receive the image means that some of the image will be cropped.
Think of it this way: you're watching a slideshow on a screen that's just big enough to accommodate the full-size image, then someone substitutes the screen with one that's smaller. Now some of the projected image (the top, bottom, and sides) "overflows" the smaller screen, so what you now see is a cropped version of the image.
You get a similar result when using a digital SLR camera with a sensor that's smaller than a piece of 35mm film -- the image cast by the lens "overflows" the sensor. Any spare space around your subject is cropped away, so the subject now fills more of the frame -- but if there's no spare space, part of the subject itself gets cropped.
This cropping or magnification effect is an advantage when using a telephoto lens, as the result is similar to using a longer lens or 1.6x converter.
However, for anyone shooting wide angle, the opposite is true. Now that awesome wide angle so prized for creating dramatic landscapes or shooting tight interiors is rendered semi useless. A genuine wide angle lens like a Canon EF 24mm gets "lengthened" by 1.6x, giving it an equivalent focal length of 38.4mm on your Rebel XSi or 50D.
Fortunately, Canon and other manufacturers of digital SLR's realized they had to produce lenses to overcome the crop factor for photographers using wide angle lenses.
The result is a range of lenses created specifically for digital SLR cameras with sensors smaller than 35mm film (often referred to as APS-C format sensors). While the crop factor still applies when using these lenses, the lenses themselves are "shorter", both physically and in focal length, than the equivalent lens for a 35mm camera.
Thus the kit lens for Canon Rebels is the EF-S 18-55mm. This would be classified as "ultra wide" (at the wide end) for a 35mm camera, but equates to a more mundane 28.8-88mm -- or standard zoom -- when used with a Rebel XSi or 50D.
While Canon's Rebels and EOS **D digital SLRs are categorized as entry-level or "prosumer" cameras, they are also used by professionals, particularly wedding photographers, who previously were not being adequately catered for by the wide angle lenses available, particularly in terms of image quality and focal length.
To overcome this, Canon introduced two lenses that are standouts within its range of EF-S lenses for use on Rebel and EOS **D cameras:
* Canon EF-S 17-55mm f/2.8 IS USM
* Canon EF-S 10-22mm f/3.5-5.6 USM
The EF-S 17-55mm is notable for its constant f/2.8 aperture throughout the zoom range which, together with image stabilization, makes it especially effective for shooting in low light without flash. It is highly rated by wedding photographers, and also as a portrait and walkaround lens.
Users like the lens for its sharp images at all focal lengths, fast autofocus, versatility and excellence in low light. Their main dislikes are the price (it's the most expensive EF-S lens), inadequate weather sealing, plus the size and weight.
It nevertheless receives a rating of 91% from users and is highly recommended as a superior quality lens for APS-C format cameras.
The EF-S 10-22mm is remarkable as it's the only true ultra wide angle for Canon Rebels and EOS **D bodies. Fit this lens on your Canon 40D and it's equivalent to a 16-35mm on a 35mm or full-frame body, offering photographers creative opportunities not otherwise possible with APS-C format cameras.
Users like the lens for its sharpness, excellent colors, fast autofocus and minimal distortion. They dislike the price, plastic construction, and lack of a provided lens hood.
But image quality is favorably compared to that of a Canon L-series lens, and users give it a rating of 90%, recommending it as essential for anyone wanting genuine wide angle capability on a Rebel or EOS **D body.
Users of Canon Rebels or EOS **D SLRs can no longer blame their kit lenses for less than stellar image quaility -- alternatives are available (albeit at a price.)
