Canon T2i Optics
Canon Rebel T2i Optics
The sub-frame sensor on the Canon Rebel T2i 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 more commonly referred to as the "focal length multiplier" since that's how it works in practice. Any lens used on the Canon T2i 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 T2i 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 T2i 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 T2i 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 Canon T2i also offers Lens Peripheral Illumination Correction, which corrects for lens shading (commonly called "vignetting"), producing uniform exposure across the frame by compensating for the light falloff seen with some lenses in the corners of the frame. Lens Peripheral Illumination Correction first appeared in the Canon 5D Mark II, and was also included in the midrange Canon 50D and 7D. This setting is enabled by default.
The "IS" in the new kit lens' 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 camera movement 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 Canon T2i's 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 sensor-shift 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 similar to 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 is a downside to body-based IS, though. Because their IS systems only affect the captured image, body-based systems don't stabilize the image seen in the optical viewfinder. (Of course, this limitation doesn't apply when 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, like you get with the Canon T2i, 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 possible 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 T2i 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 Canon T2i's center AF point. (Note that only the center AF point shows this increased accuracy at f/2.8: The surrounding AF points are all conventional f/5.6 ones.)
The AF points on the Canon T2i 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.)
While the image sensor in the Canon T2i is closely related to that in the 7D (the principle difference being fewer readout channels, yielding a lower continuous-mode frame rate), the AF system is one area where the 7D justifies its higher price. In the 7D, there's 19 AF points, including an additional cross-type (diagonal) f/2.8 AF sensor for the central point, and all points are cross-type, meaning they're sensitive to both horizontal and vertical detail. Other than its central point, all of the Canon T2i's AF points are line-type, with most being sensitive to horizontal detail, but the top-center and bottom-center ones being sensitive to vertical lines.
Like previous digital Rebels, the Canon T2i 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 do typical on-body incandescent illuminator bulbs. Following the lead of the T1i, you can disable the internal flash, yet still have the benefit of the flash-based AF-assist illuminator. You do this via the Flash Control menu, but note that this AF-illuminator-only setting for the flash head means you lose flash exposure capability until you explicitly turn it back on. If you attach a 550EX external flash unit to the Rebel T2i, its internal AF-assist illuminator can be 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 we can't verify this performance independently.)
Dust Reduction Technology
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.)
Canon T2i Optical Test Results
Kit Lens Test Results
Good performance with the Canon 18-55mm IS (Image Stabilized) kit lens.
|18mm @ f/8||55mm @ f/8|
The Canon T2i is offered with an EF-S 18-55mm f/3.5-5.6 IS kit lens, with a typical optical zoom range of ~3x. The 35mm equivalent range is about 29-88mm, because of the T2i's 1.6x "crop factor". Sharpness is good (though not tack sharp) across most of the frame at 18mm, however extreme corners are slightly softer, and there's moderate chromatic aberration and some coma distortion visible in the corners as well. Results at full telephoto are good too, with good sharpness across the frame and very low levels of chromatic aberration. All in all, a good performance for a kit lens, though the Canon T2i's 18-megapixel sensor could certainly benefit from higher-quality optics.
Note that our original copy of the kit lens was front-focusing in our Far-field shots, contributing to the overall softness in its images. The above shots have been retaken with a second copy of the 18-55mm IS lens, which did not show the same issue.
A small macro area (for an SLR kit lens), with very good detail. Flash throttled down well.
18-55mm IS Kit Lens
|Macro with Flash|
The Canon T2i's macro performance will of course depend entirely on the lens in use. With the 18-55mm IS kit lens set to 55mm, the Canon T2i captured a small (for a non-macro SLR lens) minimum area measuring just 2.47 x 1.65 inches (63 x 42 millimeters). Resolution and detail was very good and the image didn't soften as much in the corners as we are accustomed to seeing. (Most lenses have some softening in the corners at macro distances.) The Canon T1i's flash also throttled down for the macro area very well. There was no detectable shadow from the lens barrel, resulting in a good exposure with the flash, though there was a bit of light falloff toward the bottom of the frame.
Slightly higher than average barrel distortion at wide-angle, but low pincushion distortion at telephoto with the 18-55mm IS kit lens.
|Barrel distortion at 18mm is 0.9 percent|
|Pincushion distortion at 55mm is 0.1 percent|
The Canon EF-S 18-55mm IS lens produced about 0.9 percent barrel distortion at wide-angle, which is slightly above average and noticeable in some of its images. At the telephoto end, there was about 0.1% pincushion distortion, which is lower than average and hardly noticeable. Geometric Distortion 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).
The Canon T2i does not apply any geometric distortion correction to its JPEGs, as uncorrected RAW files have the same amount.
Chromatic Aberration and Corner Sharpness
Moderately high chromatic aberration at wide-angle, moderate at telephoto telephoto with the 18-55mm IS kit lens. Slight softening in some corners.
|Wide: Lower right
C.A.: Moderately high and bright
Softness: Slightly soft
|Tele: Upper right
C.A.: Moderate but dull
Softness: Slightly soft
C.A.: Very low
Softness: Slightly soft
Chromatic Aberration. Chromatic aberration in the corners with the Canon T2i's 18-55mm kit lens is pretty evident (we'd call it on the high side of "moderate") at both wide-angle (18mm) and telephoto (55mm) settings. At wide-angle, it's brighter and therefore much more noticeable than at full telephoto. Color fringing is still quite evident in the center at wide-angle, but very low in the center at full telephoto.
Corner Softness. The Canon T2i's 18-55mm IS kit lens produced some soft corners in a few shots. At full wide-angle, corners on the right side of our test targets were somewhat soft compared to the center, but the blurring didn't extended very far into the frame. The left side, however, showed less softening in the corners. The center of the image was quite sharp. At full telephoto, all four corners were only slightly soft with the top right being the softest. The image was also slightly soft in the center. An average performance for a kit lens here, but keep in mind these shots are taken wide-open and sharpness generally improves when a lens is "stopped-down" a couple of f-stops below full aperture.
The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Canon EOS Rebel T2i (EOS 550D) Photo Gallery .
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