Canon T2i Imatest Results

Canon T2i Imatest Results

We routinely use 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's some of the results produced by Imatest for the Canon T2i:

The Canon T2i showed very good color accuracy overall. Hue accuracy was really excellent (among the very best we've seen), with what little hue shift occurred taking place mostly in cyan, sky blue, red and orange. Average saturation was slightly high at 111% (oversaturated by 11%, mostly in the blues, reds, oranges and dark greens), but not unusually so for a consumer model. Average "delta-C" color error was a very low 4.04 after correction for saturation, which is excellent. All in all, a very good color response for an SLR. Mouse over the links below the illustration above to compare results with competing models.

Using the Adobe RGB color space (which provides a much wider gamut, or range of colors that can be expressed), the Canon T2i delivers more highly saturated color, as is usually the case. The average saturation was 118.8% or 18.8% oversaturated. Average saturation-corrected hue error of 5.15 "delta-C" units, which is more accurate than most of the competition here. Again, mouse over the links below the illustration above to compare results with competing models.

This image shows how the Canon T2i actually rendered the colors of the MacBeth chart, compared to a numerically ideal treatment. In each color swatch, the outer perimeter shows the color as actually captured by the camera, the inner square shows the numerically correct color after correcting for the luminance of the photographed chart (as determined by a second-order curve fit to the values of the gray swatches), and the small rectangle inside the inner square shows the numerically correct color, without the luminance correction. This image shows the Canon T2i's excellent hue accuracy, as well as a gamma curve that results in some overexposure of highly saturated swatches. (This seems to be a fairly common tactic, to produce "bright" color that's appealing to consumers, without further overdoing the saturation.)

The bottom row of the chart shows exaggerated white balance errors, revealing that the camera produced a fairly neutral color balance when using Custom white balance, with a WhiBal card as the reference.

There's a lot in this particular graph, a lot more than we really have room to go into here. (This set of plots has also changed a few revisions back in Imatest. Some of the plots that were once shown here are now shown in other Imatest output. Since we largely focus on the Noise Spectrum plot, we only show the graphic above, which includes that plot.)

In comparing these graphs with those from competing cameras, we'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.

Above, we see the results at ISO 100, which is the Canon T2i's base ISO. The luminance curve is reasonably flat, but the red, blue and green color channels exhibit much higher noise values at lower frequencies. While overall noise levels are very low, the dominance of low frequencies in the color channels points to some low-level blotchiness. Some chroma noise blotches can be seen in darker midtone and shadows of the Canon T2i's ISO 100 images, but only with extreme pixel-peeping.

Here's the same set of noise data at ISO 3,200. Here, the Noise Spectrum graph is shifted towards the left-hand, lower-frequency side than it was at ISO 100. This indicates a coarsening of the "grain" of the image noise patterns by quite a bit, and indeed that's what we see when inspecting the T2i's ISO 3,200 images. The chrominance channels' noise spectrum follows that of the luminance channel much more closely, a result of the default high ISO noise reduction applied, but is still a bit higher at very low frequencies. This can be seen in high ISO images as large but subtle blotches of chroma noise in darker areas, spread across multiple pixels. That said, the Canon T2i does a better than average job at keep chrominance noise in check.

Here's the same set of noise data at ISO 6,400. Very similar to ISO 3,200, but overall noise levels are 50-100% greater.

Here's the same set of noise data at ISO 1,2800, the Canon T2i's maximum ISO. Very similar to ISO 6,400, but even higher noise levels overall. There's also a larger shift to the left, indicating a further coarsening of the grain pattern, and the low-frequency spike in the red channel is more pronounced.

This chart compares the Canon T2i's noise performance over a range of ISOs against that of other competing cameras, using default settings. (We also compared it to the Canon 7D, since it uses a similar sensor.) While we continue to show noise plots of this sort because readers ask for them, we each time point out that the noise magnitude is only a small part of the story, the grain pattern and the amount of detail preserved are much more important. In the case of the Canon T2i, the noise magnitude starts out slightly lower than all the competition at ISO 100, and remains lower than the competition except compared to the Nikon D90 at ISOs 3,200 and 6,400, where it is slightly higher. Keep in mind these are at default settings, so the shape and positions of the curves will be influenced by your settings. The Canon T2i offers four high ISO NR settings: Disable, Low, Standard and Strong. Although it's called high ISO NR, it is applied at all ISOs.

Canon T2i 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 T2i with a nominally-exposed density step target (Stouffer 4110), and the Canon T2i's settings at their default positions, except Auto Lighting Optimization, which was set to Off.  (We'll try to come back and add results for ALO at the default "Standard" setting.)

These are good results for in-camera JPEGs, though a notch below the top performers when comparing numbers for the High quality threshold (7.53 f-stops). The tone curve shows excellent gradation in highlights but tails-off more abruptly in the shadow end. This can be seen when closely inspecting shots captured by the Canon T2i, where detail is held very nicely in strong highlights, but somewhat less so in the deepest shadows.

Processing the T2i's RAW (CR2) files through Adobe Camera Raw (ACR) version 5.7 improved dynamic range by about 0.4 f-stop at the best quality level, and almost 1.5 stops overall. ACR's automatic settings were used as a starting point, then the sliders were manually tweaked to get optimal results. Worth noting here is that ACR's default noise reduction settings reduced the pixel noise relative to the levels in the in-camera JPEG, which helps boost the higher quality Imatest scores. Also important to note is that the degree of manipulation performed by ACR here could result in color errors in strong highlights in real-world photographs. (The best performance you're likely to see in a real-world environment will likely be somewhere between the in-camera results and those from Adobe Camera Raw.)

Dynamic Range, the bottom line:

The net result was that the Canon EOS T2i's JPEGs and ACR converted RAW files scored about average in Imatest's dynamic range analysis when compared against other current APS-C sensor models. Results are very similar to the Canon 7D, with Imatest scores within about 1/3 f-stop of each other.

To get some perspective, here's a summary of the Canon T2i'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.):

Note that this test is repeatable to within 1/3 EV according to the Imatest website, so differences of less than 0.33 can be ignored.

Overall, dynamic range for the Canon T2i's in-camera JPEGs is about 1/2 stop below the best consumer-level digital SLRs, while its results from RAW files came up a stop or more low.

Canon T2i 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 2,430 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 2,325 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 2,377 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased both vertical and horizontal resolution significantly, resulting in an average of 2,188 LW/PH, a little lower than the uncorrected value, indicating the default in-camera sharpening is slightly aggressive in the horizontal direction.

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. Here, you can see that the Canon T2i's default in-camera sharpening is fairly strong (a large bump at the top end) in the horizontal direction, but almost ideal in the vertical direction. Imatest reports that the horizontal direction (vertical edge) is "oversharpened" by 7.38% while the vertical direction (horizontal edge) is "oversharpened" by just 0.08%. These numbers are probably a good default for the intended target market for the Canon T2i. That said, you should be able to extract more fine detail if you begin with a RAW file, rather than a JPEG.

Note: We don't feature SFR-based LW/PH resolution numbers more prominently in our reviews (eg, outside the Imatest pages) because we've found that they're *very* sensitive to minor differences in in-camera image processing. Relatively small changes in the amount of in-camera sharpening can have a large effect on the resulting resolution numbers. Imatest attempts to compensate for this by adjusting to a "standard" sharpening, but this approach can't completely undo what happens inside the cameras, and so often gives inconsistent results. Sometimes the "standardized" sharpening happens to just match the shape of the edge profile with the in-camera sharpening applied, and you'll get wildly high results. At other times, it will tend to correct in the opposite direction. Unfortunately, ignoring the in-camera sharpening entirely can result in even greater discrepancies, particularly between models from different manufacturers. Turning off sharpening in the camera may or may not fully eliminate the sharpening, so simply turning off sharpening in the camera JPEGs isn't a reliable solution. It also wouldn't be the way most people shoot the cameras. We could process RAW files with no sharpening (as shown below), but then that'd only suit the people working primarily or exclusively from RAW, and would open another can of worms as to what RAW converter was used, etc, etc.

The bottom line is that numbers for resolution only take you so far. Detail handling and edge acuity are very complex issues; ones that don't easily boil down to a single number. The best approach is to simply look look at the broad array of standardized test shots we take with each camera, to the point of downloading and printing them with whatever processing you'd use if you owned the camera and shot with it. See how the differences stack up for you visually, and make your decision on that, rather than on abstract resolution numbers.