Canon XTi Image Quality
Canon Rebel XTi Imaging Characteristics
Imaging and file quality are ultimately what's most important in a digital camera, and the Canon Rebel XTi definitely delivers the goods in this area. As you'll see below, Canon has taken a slightly different path in noise control with the Rebel XTi than Nikon did in the competing D80. The two cameras are surprisingly similar in their high-ISO abilities, but Canon has chosen to preserve a bit more subtle subject detail, at the cost of higher noise, while Nikon has gone the other direction, and given up some subject detail in areas of subtle contrast, but has achieved lower noise levels in the process.
For readers considering an upgrade from the original Digital Rebel, the Rebel XTi represents a very noticeable boost in resolution and overall image quality. If you're thinking of upgrading from a Rebel XT though, the step up from 8 to 10 megapixels may not be enough to justify the move on the basis of resolution alone. (Think about it: Horizontally, the XT had 3,456 pixels, the XTi has 3,888. That's a linear increase of only 12.5%. Moving up from the original Rebel's 3,072 horizontal pixels though, the XTi represents a 26.6% increase, a more noticeable improvement.)
Sensor Improvements. Canon makes its own CMOS sensors, and the images they produce are hard to beat. In the case of the 10.1 megapixel chip in the Rebel XTi, Canon claims that high-ISO performance will be very much on par with that of the Rebel XT and EOS-30D, and our tests bear that out. The pixels in the Rebel XT's sensor are on a 6.4 micron pitch, while those in the XTi's sensor are spaced only 5.7 microns apart, the smallest pixels on an EOS digital SLR to date.
Smaller pixels generally mean less light-gathering ability per pixel. If you have less light, the camera has less information to work with, and is more likely to make errors, which we call noise. To counter the potentially negative impact of the XTi's smaller pixels, Canon engineers developed a more efficient cell layout that increase the percentage of each pixel's area that is devoted to light gathering (the pixel pads are bigger). They've also improved their microlens fabrication to reduce the gap between microlenses by a factor of two over their earlier designs (each pixel has a very tiny lens to gather light and focus it on the very small pixel pad that is sensitive to light). The expected net result is that the Rebel XTi's pixels will have about the same light-gathering ability as the larger ones in the XT's sensor, despite their smaller physical dimension. We'll obviously have to wait until we can get our hands on a production sample to test, but if Canon has succeeded, the Rebel XTi's ISO 1600 performance should be every bit as good as that of the XT. This is encouraging: The practical increase in resolution in going from 8 to 10 megapixels is pretty small, but it's nice to see that it's been accomplished with no loss in noise performance.
There's no question, the Canon Digital Rebel XTi delivers impressive resolution and image quality. Here's a look at the details:
Resolution & Detail
High resolution, 1,600~1,800 lines of strong detail.
|Strong detail to
1,800 lines horizontal
|Strong detail to
1,600 lines vertical
Our laboratory resolution chart revealed sharp, distinct line patterns down to about 1,800 lines per picture height horizontally, though to only about 1,600 vertically. (The individual target lines are still resolved along the vertical axis at 1,700 lines or higher, but some significant artifacts can be seen around 1,650, so we felt that 1,600 was more representative the true resolution there.) Extinction didn't occur until past the 2,000 line point. Use these numbers to compare with other cameras of similar resolution, or use them to see just what higher resolution can mean in terms of potential detail. Beware that while you might be able to make out what looks like distinct lines at numbers higher than those we've mentioned here, the camera is just doing its best to continue interpreting the lines. So the lines you see at 2,000 and higher are really only artifacts generated by the camera's imaging system.
Sharpness & Detail
Sharp images with great detail definition. Minor edge enhancement, and minor noise suppression as well.
The Canon Rebel XTi captures nice, sharp images, with very good detail definition. Some very slight edge enhancement artifacts are visible on high-contrast subjects, such as the crop above left, though results are still very good. (Edge enhancement creates the illusion of sharpness by enhancing color and tonal differences right at the edge of a rapid transition in color or tone.)
Noise-suppression systems in digital cameras tend to flatten-out detail in areas of subtle contrast. The effects can often be seen in shots of human hair, where the individual strands are lost and an almost "watercolor" look appears. The crop above right shows some very minor detail loss to noise suppression, but detail remains fairly strong in the darkest shadows, with individual strands of hair still visible. Overall, the Canon Rebel XTi does a much better than average job of holding onto subtle detail, while reducing image noise.
Saturation & Hue Accuracy
Excellent overall color, with generally pleasing hue accuracy and saturation. Some oversaturation in strong reds, but results are still good.
Most consumer digital cameras produce color that's more highly saturated (more intense) than found in the original subjects. This is simply because most people like their color a bit brighter than life. The Canon EOS Digital Rebel XTi is guilty of pushing the strong reds somewhat, and strong blues slightly, but overall saturation is quite appealing. Where oversaturation is most problematic is on Caucasian skin tones, as it's very easy for these "memory colors" to be seen as too bright, too pink, too yellow, etc. Skin tones tended to be just slightly warm with the Rebel XTi, depending on the white balance setting, but again, results are natural and pleasing.
The other important part of color rendition is hue accuracy. Hue is "what color" the color is. The Rebel XTi did move cyan a bit toward blue and red just slightly toward orange, but overall hue accuracy is among the best we've seen to date.
|See full set of test images with explanations
See thumbnails of all test and gallery images
Exposure and White Balance
Indoors, incandescent lighting
Good color with the Manual white balance setting, but very warm results with the other options. Average positive exposure compensation required.
|Auto White Balance +1.0 EV||Incandescent WB +1.0 EV|
|Manual White Balance +1.0 EV|
Color balance indoors under incandescent lighting was very warm with both the Auto and Incandescent white balance settings, but the Manual option produced more accurate results. (Why can't manufacturers seem to design their cameras to handle household incandescent lighting better?) The Canon Rebel XTi required an average amount of positive exposure compensation here, at +1.0 EV. Overall color with the Manual white balance setting looks good, though the blue flowers are quite dark and purplish, and the greens are a bit dark as well. (Many digital cameras have trouble with the blue flowers in this shot.) Our test lighting for this shot is a mixture of 60 and 100 watt household incandescent bulbs, a pretty yellow light source, but a very common one in typical home settings here in the U.S.
Better than average exposure accuracy, though a low contrast adjustment needed to handle harsh lighting. Good overall color, if slightly dark.
|Manual White Balance,
+0.7 EV, Lower Contrast
|Auto White Balance,
Auto Exposure, Default Contrast
Outdoors, the Canon EOS Digital Rebel XTi's default contrast setting resulted in somewhat high contrast under harsh lighting, with dark shadows and hot highlights. However, the lower contrast adjustments (available via the Picture Style submenu off Record Menu 2) did a good job of toning down the image, preserving detail in both shadows and highlights without producing a muddy exposure. The portrait above left was taken with the camera's next to lowest contrast setting, which produced good-looking results, and excellent highlight detail. Overall color is natural and pleasing outdoors, if slightly dark. The camera typically required less than the average amount of positive exposure compensation. Overall, very nice results.
ISO & Noise Performance
Very low image noise, with relatively low noise even at the highest sensitivity setting.
|ISO 100||ISO 200||ISO 400|
|ISO 800||ISO 1,600|
The Canon EOS Digital Rebel XTi produced very low image noise overall. At ISO 800, noise was a little high, though without very strong blurring in the finer details. Even at ISO 1,600, noise is lower than average, and fine detail is still pretty good. As noted above, the Canon Rebel XTi does a much better job than average in suppressing image noise without losing fine/subtle subject detail.
ISO 1600 Comparison
Against its primary competitor,the Nikon D80, the Canon XTi shows more chroma noise, but better preservation of subtle detail.
While it sells at a higher price, with a more capable lens, the obvious competition for the Canon Digital Rebel XTi is Nikon's D80. In the past, Canon's noise-suppression technology has generally been superior to that of other manufacturers, but with the D80, Nikon seems to have pulled even. Given the rivalry between these two particular cameras, we'll look at two sets of crops from them both, at ISO 1,600.
In the first set of crops, we'll look at swatches from the Macbeth ColorChecker target, as an example of how each performs in areas with flat tints. We'll show both the RGB image, and the individual red, green, and blue color channels, so you can see what's going on in the individual color channels.
|ISO 1600 Flat-Tint Noise Comparison|
("Normal" high-ISO NR)
|Canon Rebel XTi|
|(click for full D80 file) -- RGB -- (click for full XTi file)|
The pixel-peeping crops above tend to favor the D80, both due to their enlargement, and because they're showing a relatively limited area. In the full images, the D80 shows some blotchiness when you look at larger areas (for instance, the dark border around the MacBeth chart) that isn't evident in the Canon Rebel XTi's image. As noted in the Imatest Results section of the D80 review, this corresponds to a spike in the very lowest frequencies on the D80's noise spectrum, compared with a notch in the same area on the Rebel XTi's spectrum plot. (We speculate that this very low-frequency blotchiness is something that Canon's active-pixel CMOS technology is particularly good at dealing with.)
In the next set of crops, we'll look at some samples from our Still Life target, where the tone-on-tone coloring of the fabric swatches reveals much about the inner details of how cameras make the trade-off between subject detail and sensor noise.
|ISO 1600 Tone-on-Tone Noise/Detail Comparison|
("Normal" high-ISO NR)
|Canon Rebel XTi|
|(click for full D80 file) ---- (click for full XTi file)|
In the first set of crops, we saw that the D80 won handily in terms of noise present in areas of flat tint. In the second set of crops, though, we see the price that's paid to achieve that, namely that fine detail in areas of subtle contrast, and some overall sharpness are lost in the process. In the crops above, we consistently see more fine detail retained by the XTi, albeit at the cost of higher image noise, particularly chroma (color) noise. It's also interesting to note how the noise processing varies as a function of the part of the color spectrum you're looking at. This is particularly evident in the D80's images, which give up detail in reds and yellows far more quickly than in blues and purples. (You can see this effect most clearly in the crop showing the red fabric swatches above.)
As always, it's important to note that we're engaged in some pretty deep pixel-peeping here, given that we're looking at images from 10 megapixel cameras 1:1 on a computer screen. Given typical computer display resolutions of ~75 pixels/inch, this corresponds to peering at printed images roughly 52 x 35 inches(!) in size. Not to totally dismiss the differences noted above though, you can see detail differences in the red fabric swatches even in 8x10 inch prints -- But you have to squint and look pretty carefully to make them out.
The bottom line is that we have two slightly different approaches to noise suppression here, one slightly favoring smoothness in flat tints, the other slightly favoring subtle detail. How you feel about either will depend a lot on your personal preferences, both cameras deliver really excellent images at high ISO settings.
Extremes: Sunlit and low light tests
Very high resolution and strong detail. Best results with the lower contrast adjustment, good overall color. Excellent low-light performance, capable of capturing bright images in near darkness.
|+0.3 EV||+0.7 EV||+1.0 EV|
Because digital cameras are more like slide film than negative film (in that they tend to have a more limited tonal range), we test them in the harshest situations to see how they handle scenes with bright highlights and dark shadows, as well as what kind of sensitivity they have in low light. The shot above is designed to mimic the very harsh, contrasty effect of direct noonday sunlight, a very tough challenge for most digital cameras. (You can read details of this test here.)
Under the harsh lighting of the test above, the Canon EOS Digital Rebel XTi produced high contrast with washed-out highlights and deep shadows with its default contrast setting. However, the Rebel XTi's next-to-lowest contrast adjustment (as seen above) produced much more pleasing results, with better midtones, without washing out the colors in the image. The Rebel XTi's contrast and saturation settings are a bit buried, down in the Picture Style sub-menu, off the main shooting menu, but they're very effective, and provide a nice range of control, with fine enough steps that you can pick exactly the effect you want. With the lower contrast setting, these images show good detail at both extremes of the tone curve, in both shadows and strong highlights. Exposure is just a little dark at the +0.7 EV compensation setting, but I felt the +1.0 EV image was too bright. (In "real life" of course, be sure to use fill flash in situations like the one shown above; it's better to shoot in the shade when possible.)
The Rebel XTi captured bright images down to the lowest light level we test at, at all ISO settings. This equates to about 1/16 the brightness of average city street lighting at night, so the XTi should be able to take well-exposed photos in any environment you can see well enough to walk around in without blundering into things. Though the images are still fairly bright at the lowest light level, the longer exposure did result in a more pronounced pink color cast. Overall noise was really quite low to begin with, but the camera's Long Exposure Noise Reduction option (Custom Function 02) did tone down the brightness of visible noise pixels. The camera's autofocus system was able to focus on the subject down to just below the 1/16 foot-candle light level unassisted, almost in line with its exposure system. (The XTi does have an autofocus-assist light option, but it uses the camera's flash tube as the illuminator, and so requires the flash system to be engaged for it to work.) Do keep in mind that the very long shutter times necessary here absolutely demand the use of a tripod or other camera support to get sharp photos. (A useful trick is to just prop the camera on a convenient surface, and use its self-timer to release the shutter. This avoids any jiggling from your finger pressing the shutter button, and can work quite well when you don't have a tripod handy.)
How bright is a foot-candle? The one foot-candle light level that this test begins at roughly corresponds to the brightness of typical city street-lighting at night. Cameras performing well at that should be able to snap good-looking photos of street-lit scenes.
NOTE: This low light test is conducted with a stationary subject, and the camera mounted on a sturdy tripod. Most digital cameras will fail miserably when faced with a moving subject in dim lighting. (For example, a child's ballet recital or a holiday pageant in a gymnasium.) Digital SLRs like the Canon XTi do much better than point & shoots, but you still shouldn't expect a quick autofocus lock with moving subjects.
Excellent print quality, great color, excellent 13x19 inch prints. ISO 1,600 images are surprisingly clean and sharp at 8x10 inches: Most users would probably find high-ISO shots acceptable even at 11x14.
Testing hundreds of digital cameras, we've found that you can only tell just so much about a camera's image quality by viewing its images on-screen. Ultimately, there's no substitute for printing a lot of images and examining them closely. For this reason, we routinely print sample images from the cameras we test on our Canon i9900 studio printer, and on the Canon iP5200 here in the office. (See the Canon i9900 review for details on that model.)
The Canon Digital Rebel XTi didn't fail to impress us when it came time to examine its printed output. JPEGs straight from the camera made good-looking 13x19 inch prints, and delivered loads of crisp detail with a little sharpening in Photoshop. Working from RAW files, you could probably print a good bit larger than 13x19 with little trouble or obvious softness.
As we've come to expect from DSLRs built around Canon's CMOS sensor technology, the Rebel XTi performed very well at high ISO settings. Shots captured at ISO 1,600 looked great printed as large as 8x10 inches, and most users would likely be satisfied with prints as large as 11x14 inches at that ISO setting. We did notice some chroma (color) noise in areas of flat tint, but it wasn't too bad at print sizes of 8x10 inches or below. As noted elsewhere in this review, when compared against its primary competitor the Nikon D80, the XTi tends to have a bit more chroma noise than the D80, but conversely does a better job of holding onto subject detail in areas of subtle contrast, or tone-on-tone color variations.
Noise is an awfully subjective issue though: What you think of the XTi's ISO 1,600 shots will depend a lot on your tolerance for image noise, and the size you intend to print at. We ourselves tend to print at 8x10 or 8.5x11 inches, frame the results, and hang them on a wall or park them on a table. With that set of givens, we'd feel perfectly comfortable shooting with the XTi at ISO 1,600 as a matter of course. We don't say that often.
The XTi's color was also beautiful, as printed on the i9900. Strong reds did tend to saturate somewhat, but not as badly as on many cameras, and the rest of the spectrum was rendered very nicely. Skin tones were very natural, and the whole effect was bright, but very believable and appealing. Bottom line, the XTi delivered excellent prints.
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