Canon XSi Optics
Canon XSi Optics
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.)
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
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.)
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.)
Pretty good performance with the 18-55mm IS (Image Stabilized) kit lens.
The Canon XSi comes with an 18-55mm IS kit lens, which is a fairly average optical zoom range of ~3x for a kit lens. Details are reasonably sharp, with a hint of softness across the frame at maximum wide-angle, with relatively low levels of coma distortion but some noticeable chromatic aberration and slight blurring in the extreme corners. Results at full telephoto are even better, with good sharpness across the frame and very low levels of chromatic aberration. All in all, while not up to the performance of top-level zooms, it does pretty well for a kit lens on a 12-megapixel sensor, and the added image stabilization will come in handy for low-light shooting.
A small area (for an SLR kit lens), with excellent detail. Flash throttles down well.
|Standard Macro with 18-55mm IS |
|Macro with Flash|
As with zoom performance, the Canon XSi's macro performance will depend entirely on the lens in use. However, with the 18-55mm IS kit lens set to 55mm, the XSi captured a small (for a non-macro SLR lens) minimum area measuring 2.46 x 1.64 inches (62 x 42 millimeters). Resolution and detail were high, though with some softening in the corners from the lens. (Most lenses have some softening in the corners at macro distances.) The Canon XSi's flash throttled down for the macro area pretty well, and there was no detectable shadow from the lens barrel, resulting in a good exposure with the flash.
Moderate barrel distortion with the 18-55mm IS kit lens, though low pincushion.
|Barrel distortion at 18mm is 0.8 percent|
|Pincushion at 55mm is less than 0.1 percent|
The Canon XSi's 18-55mm IS kit lens produced about 0.8 percent barrel distortion at wide-angle, which is about average among the cameras we've tested, and noticeable in its images. At the telephoto end, the less than 0.1 percent pincushion distortion is quite low and not nearly as noticeable. This is the tendency for the lens to bend straight lines outward (like a barrel -- usually at wide-angle) or inward (like a pincushion -- usually at telephoto).
Moderate and bright at wide-angle, but lower at telephoto with the 18-55mm IS kit lens.
|Wide: Moderate but bright, |
top left @ 200 percent
|Wide: Quite bright, |
top right @ 200 percent
|Tele: Low and dull, |
top left @200 percent
|Tele: Low, |
top right @200 percent
Chromatic aberration in the corners with the Canon XSi's kit lens is pretty evident (we'd call it on the high side of "moderate," or "noticeable") at the 18mm setting. It's less distinguished by its width (6-8 pixels) and more by its brightness. Remember, though, that this is at 12 megapixels, so its width diminishes relative to this high resolution. At 55mm telephoto, this distortion is much lower and hardly noticeable. (This distortion is visible as a slight colored fringe around the objects at the edges of the field of view on the resolution target.)
Minimal blurring in the corners of the frame at both zoom settings with the kit lens.
|Wide: Slightly soft in the |
corners (upper left).
|Wide: Sharper at center.|
|Tele: Slightly soft in the |
corners (upper left)
|Tele: Sharper at center.|
The Canon XSi's 18-55mm IS kit lens produced slightly soft corners in a few shots. At wide angle, corners on our test targets had very little softness compared to the center. At telephoto, corners showed slightly more softness, but still much less than average for a kit lens. A very good performance here, especially considering these shots were taken at maximum aperture (corner softness usually improves when the lens is stopped-down a few f-stops). (We did see a bit more corner softness at wide angle in our Far-Field shot of the house, but that could also have been a matter of the background in the corners being a good bit more distant than the main focal point at the center of the frame.) Impressive results.
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 .
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