Canon XSi Optics

Canon XSi Optics

Kit Lens

Among the bigger hardware improvements is the inclusion of the new EF-S 18-55mm IS lens in the Canon XSi kit. Though this light, compact image-stabilized lens sells for $200 on its own, the Canon XSi kit will sell for only $100 more than the body-only price. In our testing over at SLRgear.com, this lens has really impressed over past versions of the standard Digital Rebel kit lens, with improved sharpness and reduced chromatic aberration. It's Canon's answer to the competing SLRs from Olympus, Pentax, Samsung, and Sony that use body-based, sensor-shift image stabilization, while Nikon and Canon use optical image stabilization.

Lens Compatibility

Like other Canon DSLRs with sub-frame sensors (currently, the Canon Rebel XSi, XTi, the just-announced XS, and Canon EOS-30D, 40D), the Canon Rebel XSi will work with pretty much any EF-mount lens ever made, as well as with the special EF-S lenses designed for cameras with APS-C size sensors. Designed with a smaller image circle (the area covered by the image on the film/sensor plane), EF-S lenses tend to be smaller and lighter than full-frame models with the same focal length and maximum aperture. EF-S lenses can't be used on full-frame Canon cameras (nor on their models with 1.3x crop factors, like the current EOS-1D Mark III), but small-sensor cameras like the Rebel XSi can use any full-frame lenses in Canon's arsenal.

The sub-frame sensor on the Canon Rebel XSi means that it has a smaller angle of view (by a factor of 1/1.6) than a full-frame camera with any given lens. While most properly called a "crop factor," the 1.6x ratio is most commonly referred to as the "focal length multiplier" since that's how it works in practice: Any lens used on the Rebel XSi will have the same field of view as one with a 1.6x greater focal length would when attached to a 35mm camera. (For example, a 100mm lens on the XSi will show the same field of view as a 160mm lens on a camera with a 35mm frame size.) This means that the 18-55mm kit lens for the XSi has a coverage roughly equivalent to that of a 29-88mm lens on a 35mm camera.

The 18-55mm f/3.5-5.6 IS lens that ships with the Canon XSi is of only modest quality in terms of build, like most kit lenses, but optically it's pretty impressive, producing good focus fields from wide open to f/16. Still, a better lens is one of the first accessories new SLR owners typically buy. (See our sister site SLRgear.com for in-depth lab tests and user reviews of SLR lenses.)

Image Stabilization

The "IS" in the new kit lens's name stands for "Image Stabilization." This is a handy and increasingly common feature of both lenses and some camera bodies. The idea is that a motion sensor, computer chip, and movable optical element are built into the lens barrel. The motion sensor detects motion that could lead to blurred images with long exposure times. This data is processed by the computer chip, which in turn moves the floating optical element in such a way as to counteract the motion of the lens, thereby keeping the image stationary on the camera's image sensor. The net result is that you can shoot at much slower shutter speeds than you'd otherwise be able to, without getting blurred images as a result.

Image stabilization technology generally works very well, and Canon has been making IS equipment longer than most competing manufacturers. The net result is that the new IS-enabled kit lens does a great job of delivering sharp photos under dim lighting: In the case of the 18-55mm f/3.5-5.6 IS lens, you should be able to shoot at shutter speeds up to 16x (4 stops) slower than you can normally hand-hold without IS.

As mentioned above, some manufacturers have begun building Image Stabilization capability into their camera bodies, rather than their lenses. The advantage of this is obvious: Every lens you put on a camera with body-based stabilization becomes an IS lens. A body-based IS system makes for a much less expensive lens collection, since you don't have to pay for IS technology in every lens you buy. There are two downsides to body-based IS, though. First, we've found that the best lens-based IS systems outperform the best body-based IS systems by a factor of two or more (one f-stop). Second, the body-based systems don't stabilize the image seen in the viewfinder, unless you're using a Live View camera that captures its viewfinder image from the camera's main sensor. In extreme cases, having a stabilized view through the viewfinder can be a great aid to accurate framing and improving your own stability for sharper pictures.

So neither IS technology (lens-based or body-based) wins over the other on all points. If you plan on assembling a large collection of lenses, body-based IS will save you money, but at some cost in performance, and without the benefit of a stabilized view through the viewfinder. Lens-based IS can work better (not all IS systems are created equal), but will cost you more if you end up buying a lot of lenses. For the casual user who doesn't plan on acquiring a large lens collection, either approach will work, now that IS-enabled kit lenses are appearing as part of consumer-level SLR bundles.

Canon XSi Autofocus

The Rebel XSi uses a new sensor with enhanced precision a the center point. The new design achieves more precise focus when used with lenses with apertures of f/2.8 or larger.

SLR autofocus accuracy is governed in part by how far apart you can space the sensor elements for a single AF point from each other. The wider this "baseline," the more accurately the AF point can determine focus. What limits the AF baseline spacing in a camera system is the lens aperture. You can build sensors with wider baselines, but that also restricts the range of lenses they can be used with. By and large, camera manufacturers have set f/5.6 as the minimum aperture their AF systems will work with. Lenses with wide-open apertures smaller than f/5.6 just won't focus. But once the baseline spacing is set, using a faster lens doesn't improve focus accuracy any. That is, unless you add a larger baseline sensor, as is built into the Rebel XSi.

On their higher-end DSLRs, Canon uses special AF sensors for some of the AF points. These dual-precision AF points have two sets of focus sensors, one spaced to permit use with f/5.6 lenses, the other set to operate with f/2.8 lenses. Since an f/2.8 lens permits almost a 2x increase in baseline spacing, AF precision increases proportionately when using them.

The AF points on the Rebel XSi cover about 60% of the frame width, and about 50% of the frame height. The illustration above shows the relative spacing between the various AF points, with the distances between them marked in millimeters, at the focal plane. (Illustration courtesy Canon USA.)

AF-Assist Illuminator

Like previous digital Rebels, the XSi uses the built-in flash head as its AF-assist illuminator, rather than a bright light built into the camera's body. This works well because the flash is quite bright, and probably has a greater range than an on-body illuminator bulb. Unlike other models in the Rebel line, you can disable the internal flash by going into the Flash Control menu, which still lets the AF-assist pulse to fire; but then you lose your flash capability until you turn it back on. If you attach a 550EX external flash unit to the Rebel XSi, its internal AF-assist illuminator canbe used instead of the flash head itself, providing a useful working range of about 50 feet with a less obtrusive light source. For non-flash photography, Canon's ST-E2 wireless sync transmitter can also be used for AF assist, a handy trick. The ST-E2's AF-assist light has a useful range of about 25 feet. (Note though, that we haven't tested the ST-E2 ourselves, so can't verify this performance independently.)

Canon XSi Anti-Dust Technology

The Rebel XSi digital SLR camera includes Canon's EOS Integrated Cleaning System, first introduced on the EOS Rebel XTi camera. The camera's Self-Cleaning Sensor Unit shakes dust particles off of the low-pass filter in front of the sensor. The dust is then trapped by an adhesive along the base, preventing it from causing further nuisance. Cleaning is engaged each time the camera is powered up or shut down, or manually through the "clean now" function.

The second part of the cleaning system involves post processing with a compatible personal computer and the supplied Digital Photo Professional software. Here (via a menu option) the camera maps any spots that may remain on the sensor, saving it as Dust Delete Data and subsequently subtracting dust spots from the final image during post processing. A third option includes a manual sensor cleaning function which raises the mirror and allows users to clean dust that may have stuck to the low-pass filter.

Despite the new dust cleaning features, we haven't seen an automatic system capable of removing all dust. So while this is a nice feature to have, don't be fooled into thinking that you won't have to either learn how to clean your sensor or send it in for cleaning.

Everyone understands that lenses sometimes get dust on them and need to be cleaned, and there are a lot of lens-cleaning cloths, solutions and other accessories on the market that work well. But what do you do when your sensor gets dusty? Dust specks on the sensor tend to show up when shooting at very small apertures, appearing as dark blobs on your images. They're distracting at best, a terrible nuisance at worst, if you end up having to retouch every image to be rid of them.

Most of us are naturally leery of the idea of poking around inside the delicate innards of our DSLRs to wrestle with recalcitrant dust specks. Gently blowing the sensor surface (actually, the surface of the anti-aliasing filter) with compressed air gets rid of some dust, but there's invariably a lot that just stays stuck, no matter what. So what do you do?

If you've got dust specks on your sensor (and sooner or later you will), you're going to need to clean it. There are a lot of products out there intended to address this need, but a distressing number of them work poorly (if at all), and many are grossly overpriced. Advertising hype is rampant, with bogus pseudo-scientific jargon and absurd product claims. And prices - Did we mention prices? How about $100 for a simple synthetic-bristle brush?

So how do you know which product to use?

We don't pretend to have used everything currently on the market, we but can tell you about one solution that worked very well for us. The "Copper Hill" cleaning method is straightforward and safe, and in our routine usage here at Imaging Resource, very effective. Better yet, the products sold by Copper Hill Imaging are very reasonably priced. Best of all, Nicholas R (proprietor of Copper Hill) has put together an amazingly detailed tutorial on sensor cleaning, free for all.

Sensor cleaning is one of the last things people think about when buying a DSLR, but it's vital to capturing the best possible images. Take our advice and order a cleaning kit from Copper Hill right along with your DSLR, so you'll have it close at hand when you need it: You'll be glad you did!

(While they've advertised on our sister site SLRgear.com from time to time, we receive no promotional consideration from Copper Hill for this note. We just think their sensor cleaning products are among the best on the market, and like their way of doing business. -- We think you will too. Check them out.)

The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Canon EOS XSi (Rebel XSi, Canon 450D) Photo Gallery .