Canon XTi Imatest Results

Canon Digital Rebel XTi Imatest Results

We routinely Norman Koren's excellent "Imatest" analysis program for quantitative, thoroughly objective analysis of digicam test images. I highly recommend it to our technically-oriented readers, as it's far and away the best, most comprehensive analysis program I've found to date.

My comments below are just brief observations of what we see in the Imatest results. A full discussion of all the data Imatest produces is really beyond the scope of this review: Visit the Imatest web site for a full discussion of what the program measures, how it performs its computations, and how to interpret its output.

Here are some of the results produced by Imatest for the Canon Rebel XTi:

The Canon XTi showed good color accuracy overall, but went a bit overboard with strong reds. This is a fairly common response with this target (the MacBeth ColorChecker), and the XTI's response is fairly typical for consumer DSLRs, more highly saturated than that of pro models. Hue accuracy is excellent, with minor shifts in a number of colors, rather than large shifts in any one area. The largest shift was in yellow-orange, which moved toward yellow a fair bit. Average saturation was 109.7% (oversaturated by 9.7%, largely in the reds, but somewhat in the blues as well), average "delta-E" color error was just 4.7 after correction for saturation. All in all, an excellent, nicely-balanced color response for a consumer SLR, one of the best we've seen to date. (The Canon Rebel XTi beats its main competitor the Nikon D80 slightly in hue accuracy, and is a bit less saturated in strong reds and blues.)

This image shows how the Canon Rebel XTi actually rendered the colors of the Macbeth chart, compared to a numerically ideal treatment. (Apologies for the tilt, the Macbeth chart on our Multi target had slipped slightly in these shots.) In each color swatch, the outer perimeter shows the color as actually captured by the camera, the inner square shows the color after correcting for the luminance of the photographed chart (as determined by a 2nd-order curve fit to the values of the gray swatches), and the small rectangle inside the inner square shows what the color should actually be, based on perfect rendering in the sRGB color space. This image shows the excellent hue accuracy, as well as a gamma curve that results in an overexposure of highly saturated swatches.

As always, there's a lot in this particular graph, a lot more than we have room to go into here. In comparing these graphs with those from other competing cameras, I've found that the Noise Spectrum graph at lower right is the most important. Cameras that manage to shift their noise spectrum to higher frequencies have much finer-grained noise structures, making their noise less visually objectionable. In the graph above, this would show up as a noise spectrum curve that remained higher on the right side, representing higher noise frequencies. The champion at this was the Canon EOS-1Ds Mark II, which produced remarkably fine-grained image noise, even at very high ISOs. Bottom line, the Canon XTi's noise levels are low at low ISO, and the frequency spectrum somewhat favors high frequencies, but there's a spike at very low frequencies that would normally translate into a blotchy appearance, were the overall level of the noise high enough to be seen (it isn't).

Here's the same set of noise data at ISO 1600. Here, more of the mass of the Noise Spectrum graph is shifted toward the left-hand, lower-frequency side than it was at ISO 100, coarsening the "grain" of the image noise patterns. Not bad, but there's quite a lot of noise energy present in the left-hand side of the graph, from 0.0 to 0.2 cycles/pixel, rather than in the range from 0.2 to 0.5 cycles/pixel. This is visible in its images, where the XTi's luminance noise is generally higher than that of the Nikon D80. (There is very interesting and subtle difference in the noise spectra of the two cameras though: The D80's has a spike leading right up to zero frequency, whereas the XTi has a dip there. This shows up in high ISO shots as a slightly "splotchy" look to large, uniform dark areas in the D80's images that isn't shown by the XTi's. (You can see this in the black surround on the MacBeth chart in our Multi Target shots.

This chart compares the Canon XTi's noise performance over a range of ISOs against that of other cameras. While I continue to show noise plots of this sort because readers ask for them, I each time point out that the noise magnitude is only a small part of the story, the grain pattern being much more important. There's also the matter of how much subtle subject detail is traded away in pursuit of low noise figures. In the case of the Canon Rebel XTi, while it shows slightly better than average noise levels at higher ISO, it does let through quite a bit more noise than does the Nikon D80 under similar conditions. The upside to this is that the XTi does a much better job of holding onto fine subject detail in areas of subtle contrast. (For instance, check out the red fabric swatch with the black leaves in our Still Life test shot, and compare the detail there with that from the Nikon D80.)

Dynamic Range Analysis

A key parameter in a digital camera is its Dynamic Range, the range of brightness that can be faithfully recorded. At the upper end of the tonal scale, dynamic range is dictated by the point at which the RGB data "saturates" at values of 255, 255, 255. At the lower end of the tonal scale, dynamic range is determined by the point at which there ceases to be any useful difference between adjacent tonal steps. Note the use of the qualifier "useful" in there: While it's tempting to evaluate dynamic range as the maximum number of tonal steps that can be discerned at all, that measure of dynamic range has very little relevance to real-world photography. What we care about as photographers is how much detail we can pull out of the shadows before image noise becomes too objectionable. This, of course, is a very subjective matter, and will vary with the application and even the subject matter in question. (Noise will be much more visible in subjects with large areas of flat tints and subtle shading than it would in subjects with strong, highly contrasting surface texture.)

What makes most sense then, is to specify useful dynamic range in terms of the point at which image noise reaches some agreed-upon threshold. To this end, Imatest computes a number of different dynamic range measurements, based on a variety of image noise thresholds. The noise thresholds are specified in terms of f-stops of equivalent luminance variation in the final image file, and dynamic range is computed for noise thresholds of 1.0 (low image quality), 0.5 (medium image quality), 0.25 (medium-high image quality) and 0.1 (high image quality). For most photographers and most applications, the noise thresholds of 0.5 and 0.25 f-stops are probably the most relevant to the production of acceptable-quality finished images, but many noise-sensitive shooters will insist on the 0.1 f-stop limit for their most critical work.

The image below shows the test results from Imatest for an in-camera JPEG file from the Canon XTi with a nominally-exposed density step target (Stouffer 4110), with the Canon Rebel XTi's contrast setting at its default position.

These are decent if not exceptional numbers for an in-camera JPEG, but the dynamic range number at "high" quality itself (7.04) isn't the most important thing to look at here. A dynamic range of 7.04 f-stops at high image quality is within a half a stop of most of the XTi's competition, not enough difference to be a significant issue. (Yes, a half a stop more dynamic range would be nice, but that's not the main thing that affects what a photographer will perceive as the XTi's dynamic range here.)

More significant than the absolute dynamic range is the shape of the density curve, and how smoothly it transitions from highlight to midtone to shadow. The XTi has a very smooth curve at the highlight (right) end of its curve, and a smooth but slightly more abrupt curve at the shadow (left) end. This tells us that the XTi does a good job of holding both highlight and shadow detail, but slightly better at the highlight end of the tonal scale.

That's the story with JPEG files created in the camera, how about in an external RAW converter, such as Adobe Camera Raw (ACR)?

Using its automatic settings (which in my experience has generally worked best for dynamic range tests like this), ACR managed to extract a very healthy 8 tenths of a stop more dynamic range from the CR2 RAW file at the highest quality level, while still maintaining very good shape in the tone curve. This boost in dynamic range is enough to put the XTi's performance under ACR close to the top of the pack, regardless of camera selling price.

Dynamic Range, the bottom line:

So what conclusions can we draw about the Canon Rebel XTi's dynamic range? I think two things: First, that the XTi's internal JPEG output produces a nicely shaped tone curve, with good detail available at both ends of the curve. Beyond that though, the XTi's CR2 RAW files do contain a modest amount of additional information that the right RAW converter can extract.

To get some perspective, here's a summary of the Canon Rebel XTi's dynamic range performance, and how it compares to other digital SLRs that we also have Imatest dynamic range data for. (Results are arranged in order of decreasing dynamic range at the "High" quality level.):

Standard Observations:
The results shown in the table are interesting. One of the first things that struck me when I initially looked at test data for a wide range of d-SLRs, was that here again, purely analytical measurements don't necessarily correlate all that well with actual photographic experience. There's no question that the Fuji S3 Pro deserves its place atop the list, as its unique "SR" technology does indeed deliver a very obvious improvement in tonal range in the highlight portion of the tonal scale. I was surprised to see the analytical results place the original Olympus EVOLT as highly as they did, given that our sense of that camera's images was that they were in fact noisier than those of many other d-SLRs that we looked at. In the other direction, I was quite surprised to see the Nikon D2x place as low on the listings as it did, given that we found that camera's shadow detail to be little short of amazing.

One thing that's going on here though, is that we tested each camera at its lowest ISO setting, which should produce best-case noise levels. This is in fact what many photographers will be most interested in, but it does perhaps place some of the Nikons at a disadvantage, as their lowest ISO setting is 200, as compared to the ISO 100 settings available on most other models.

Canon Rebel XTi Specifics:
Regardless of the positions of the other cameras though, the Canon XTi does appear to have a good dynamic range, coupled with a well-proportioned tone curve. And working with its RAW files via Adobe Camera Raw (we used version 3.6 in these tests), it delivers really excellent dynamic range by any standard.

As I always say though, at the end of the day, I think you have to take the figures here with a grain of salt, and look at actual images with your own eyes to see what you make of each camera's tonal range and noise levels. We'll continue performing these dynamic range tests on the digital SLRs that we review, but (just as with the laboratory resolution target results), we suggest that you not rely on them exclusively for making your purchase decisions.

Resolution Chart Test Results

The chart above shows consolidated results from spatial frequency response measurements in both the horizontal and vertical axes. The "MTF 50" numbers tend to correlate best with visual perceptions of sharpness, so those are what I focus on here. The uncorrected resolution figures are 1611 line widths per picture height horizontally and 1612 lw/ph vertically. Correcting to a "standardized" sharpening with a one-pixel radius increases these numbers slightly, to an average of 1757 LW/PH. It appears that Canon has struck a very good balance between the XTi's anti-aliasing filter (which blurs the image somewhat, to avoid digitization artifacts) and its in-camera image sharpening. The XTi's images show good sharpness right from the camera for normal-sized prints (up to about 13x19 inches), but as always, you can do better with finely-tuned sharpening in Photoshop or another image-processing application, particularly if you're going to be printing at very large sizes.

To see what's going on, refer to the plots below, which show the actual edge profiles for both horizontal and vertical edges, in both their original and corrected forms. Bottom line, the XTi captures really excellent detail, really making the most of its 10.1 megapixel sensor. You can't sharpen its camera-created JPEGs quite as much as those from some of Canon's professional DSLRs, but they're much crisper looking straight from the camera, as is appropriate for a DSLR aimed at the consumer market. That said, enthusiast shooters will be able to extract a lot of fine detail with a little playing around using Photoshop's unsharp masking operator or other image-sharpening application.