The previous section helped you sort out what lens you need to buy with your XTi (assuming you already didn't own any Canon lenses). Now, you're probably wondering what lenses can be added to your growing collection. (Trust me, it will grow.) You need to know which lenses are suitable and, most importantly, which lenses are fully compatible with your Digital Rebel XTi.
With the Rebel XTi, the compatibility issue is a simple one: it accepts any lens with the EF or EF-S designation, with full availability of all autoaperture, autofocus, autoexposure, and image-stabilization features (if present). It's comforting to know that any EF (for full-frame or cropped sensors) or EF-S (for cropped sensor cameras only) will work as designed with your camera.
In addition to EF and EF-S lenses, you can also attach Nikon F mount, Olympus OM, Leica R, and M42 ("Pentax screw mount") and some older Canon lenses using the previous lens mounting system with a simple adapter, if you don't mind losing automatic focus and aperture control. If you use one of these lenses, you'll need to focus manually (even if the lens operates in autofocus mode on the camera it was designed for), and adjust the f/stop to the aperture you want to use to take the picture (because you lose the autoaperture feature, as well).
With so many automatic features available in lenses, it's easy to get confused. They've been available for Canon cameras for so long that we tend to take them for granted. Even so, you need to have a clear understanding of these capabilities, because some lenses and accessories don't have them. Here's a quick refresher.
■ Auto aperture. The lens remains set to its maximum (wide open) aperture while you are viewing and composing the image. When the shutter release is pressed, the aperture automatically changes (stops down) to the f/stop used to expose the image (the taking aperture). This enables you to compose your image using the brightest possible view, and also at the f/stop that provides the least amount of depth-of-field, so you can better estimate whether or not the sharply focused (and out-of-focus) parts of the image are what you want. You can also stop down the lens to the taking aperture by pressing the depth-of-field button. If you use older lenses (with an adapter) that have aperture rings, or so-called preset lenses, which were a type of lens with a manual aperture that you always had to stop down manually to the preset taking aperture, you don't get the benefit of auto aperture operation.
■ Autoexposure. When the Rebel XTi is set to a Basic Zone or Creative Zone exposure mode other than Manual, the camera can tell the lens which f/stop to use for automatic exposure. Usually the aperture is selected by the camera, but it can be chosen by you in Aperture Priority mode.
■ Autofocus. Autofocus is the ability of the Rebel XTi to set sharp focus for you, although you can also manually focus when the lens is set to the M position, or you are using a lens that does not have autofocus capabilities (with an adapter; all EF and EF-S lenses have electronic focus).
That means that lenses that don't have an aperture ring (such as Nikon G-series lenses) must be used only at their maximum aperture. Because of these limitations, you probably won't want to make extensive use of "foreign" lenses on your XTi, but an adapter can help you when you really, really need to use a particular focal length but don't have a suitable Canon-compatible lens. For example, I occasionally use an older 400mm lens that was originally designed for the Nikon line on my XTi. The lens needs to be mounted on a tripod for steadiness anyway, so its slower operation isn't a major pain. Another good match for me is the 105mm Micro-Nikkor that I sometimes use with my Rebel XTi (see Figure 6.3). Macro photos, too, are most often taken with the camera mounted on a tripod, and manual focus makes a lot of sense for fine-tuning focus and depth-of-field. Because of the contemplative nature of close-up photography, it's not much of an inconvenience to stop down to the taking aperture just before exposure.
The limitations on use of lenses within Canon's own product line (as well as lenses produced for earlier Canon SLRs by third-party vendors) is fairly clear-cut. The Digital Rebel XTi cannot be used with any of Canon's earlier lens mounting schemes for its film cameras, including the immediate predecessor to the EF mount, the FD mount (introduced with the Canon F1 in 1964 and used until the Canon T60 in 1990), FL (1964-1971), or the original Canon R mount (1959-1964). That's really all you need to know. While you'll find FD-to-EF adapters for about $40, you'll lose so many functions that it's rarely worth the bother.
In retrospect, the switch to the EF mount seems like a very good idea, because the initial EOS film cameras can now be seen as the beginning of Canon's rise to eventually become the leader in film and (later) digital SLR cameras. By completely revamping its lens mounting system, the company was able to take advantage of the latest advances in technology without compromise.
For example, when the original EF bayonet mount was introduced in 1987, the system incorporated new autofocus technology (EF actually stands for "electro focus") in a more rugged and less complicated form. A tiny motor was built into the lens itself, eliminating the need for mechanical linkages with the camera. Instead, electrical contacts are used to send power and the required focusing information to the motor. That's a much more robust and resilient system that made it easier for Canon to design faster and more accurate autofocus mechanisms just by redesigning the lenses.
WHY SO MANY LENS MOUNTS?
Four different lens mounts in 40-plus years might seem like a lot of different mounting systems, especially when compared to the Nikon F mount of 1959, which retained quite a bit of compatibility with that company's digital camera bodies during that same span. However, in digital photography terms, the EF mount itself is positively ancient, having remained reasonably stable for almost two decades. Lenses designed for the EF system work reliably with every EOS film and digital camera ever produced.
However, at the time, yet another lens mount switch, especially a change from the traditional breech system to a more conventional bayonet-type mount, was indeed a daring move by Canon. One of the reasons for staying with a particular lens type is to "lock" current users into a specific camera system. By introducing the EF mount, Canon in effect cut loose every photographer in its existing user base. If they chose to upgrade, they were free to choose another vendor's products and lenses. Only satisfaction with the previous Canon product line and the promise of the new system would keep them in the fold.
Today, in addition to its EF lenses, Canon offers lenses that use the EF-S (the S stands for "short back focus") mount, with the chief difference being (as you might expect) lens components that extend farther back into the camera body of some of Canon's latest digital cameras (specifically those with smaller than full-frame sensors), such as the XT and XTi. As I'll explain next, this refinement allows designing more compact, less-expensive lenses especially for those cameras, but which don't fit models like the EOS 5D, IDs Mark II, or 1D Mark III and 1D Mark II N (even though the latter two cameras do have a sensor that is smaller than full frame).
Canon's EF-S lens mount variation was born in 2003, when the company virtually invented the consumer-oriented digital SLR category by introducing the original EOS 300D/Digital Rebel, a dSLR that cost less than $1,000 with lens at a time when all other interchangeable lens digital cameras (including the XTi's "grandparent," the original EOS 10D) were priced closer to $2,000 with a basic lens. Like the EOS 10D introduced earlier that same year, the Digital Rebel featured a smaller than full-frame sensor with a 1.6X crop factor (Canon calls this format APS-C). But the Digital Rebel accepted lenses that took advantage of the shorter mirror found in APS-C cameras, with elements of shorter focal length lenses (wide angles), that extended into the camera, space that was off limits in other models because the mirror passed through that territory as it flipped up to expose the shutter and sensor.
In short (so to speak), the EF-S mount made it easier to design less-expensive, wide-angle lenses that could be used only with 1.6X-crop cameras (they can't be mounted on other Canon cameras), and featured a simpler design and reduced coverage area suitable for those non-full-frame models. The new mount made it possible to produce lenses like the ultra-wide EF-S 10-22mm f/3.5-4.5 USM lens, which has the equivalent field of view as a 16mm-35mm zoom on a full-frame camera.
Suitable cameras for EF-S lenses include the original Digital Rebel/300D, the Digital Rebel XT/350D, the Canon EOS 20D/30D, and, of course, the Digital Rebel XTi/400D. The EF-S lenses cannot be used on the EOS 10D, the 1D Mark II N (which has a 28.7mm X 19.1mm APS-H sensor with a 1.3X crop factor), or any of the full-frame digital or film EOS models. It's easy to tell an EF lens from an EF-S lens: the latter incorporate EF-S into their name! Plus, EF lenses have a raised red dot on the barrel that is used to align the lens with a matching dot on the camera when attaching the lens. EF-S lenses and compatible bodies use a white square instead. EF-S lenses also have a rubber ring at the attachment end that provides a bit of weather/dust sealing and protects the back components of the lens if a user attempts to mount it on a camera that is not EF-S compatible.
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