Canon 50D Imatest Results

Canon 50D 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 50D:

The Canon 50D showed very good color accuracy, though with saturation levels a little higher than some competing models. Hue accuracy was very good, with only minor hue shifts occurring mainly in the oranges and reds. Average saturation was 109.7% (9.7% oversaturated) and average "delta-C" color error was only 4.55 after correction for saturation, which is very good. (Delta-C is the same as the more commonly referred to delta-E, but delta-C takes into account only color differences, ignoring luminance variation.) Over much of the spectrum, the 50D's default color rendering is quite faithful to the original, with accurate saturation levels. The only significant deviation is in warm-hued colors, most particularly reds, tapering off as you move through oranges to yellows. The color error plot has such a distinctive shape to it that we assume this must be the result of a deliberate engineering decision. Perhaps consumers judge the vibrancy of an image more by how bright the red through yellow colors are? We're not sure of the answer to that, but we personally found the Canon 50D's color very appealing. Overall, a very good color response for an SLR. Mouse over the links below the illustration above to compare results with other recent models.

As is true of most SLRs, when using the Adobe RGB color space (which provides a much wider gamut, or range of colors that can be expressed), the Canon 50D delivers more highly saturated color, with an average saturation of 114.8% and average saturation-corrected hue error of 5.86 "delta-C" units. The bias toward higher saturation in the warm-hued colors is even more apparent here, but what was a slight undersaturation of the cyan swatch in sRGB color space is now a noticeable desaturation in Adobe RGB. Again, mouse over the links below the illustration above to compare results with other recent prosumer SLRs.

This image shows how the Canon 50D 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 very good hue accuracy, as well as a gamma curve that results in moderate overexposure of some of the highest-saturation swatches. (This increase in luminance for the highly-saturated swatches seems to be a fairly common tactic, to produce "bright" color that's appealing to consumers, without further overdoing the saturation.)

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

In comparing these graphs with those from 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 (and still is) the Canon EOS-1Ds Mark II, which produced remarkably fine-grained image noise, even at very high ISOs.

The Canon 50D does a pretty good job of keeping plenty of the luminance noise energy (indicated by the black line) at high frequencies. What little low-ISO image noise that's there is more fine-grained as a result. Chroma noise, though, is much higher, especially in the red channel. The overall noise levels are so low in low-ISO shots that the chroma noise in general and the red blotchiness in particular are only slightly visible when inspecting shadow areas of low ISO shots very closely. Canon says their High ISO Noise Reduction (the default setting is "Standard" for the 50D) is also effective at reducing shadow noise at low ISOs, which may explain why chroma noise is lower than the 40D which defaults to "Off" for its High ISO NR.

Above is the same set of noise data at ISO 3,200. Here, the Noise Spectrum graph is shifted quite a bit toward the left-hand, lower-frequency side than it was at ISO 100, coarsening the "grain" of the image noise patterns quite a bit. The red, blue and green channels still exhibit more noise at the low end of the frequency spectrum, but unlike the 40D, they track luminance noise quite nicely otherwise. Again, because the Canon 50D's High ISO Noise Reduction is enabled by default, it eliminates much of the chroma noise we saw in the plots from the 40D.

Here's the same set of noise data at ISO 6,400. While the spectrum graph is very similar to ISO 3,200 (just a slight shift to higher frequencies), note the increase in noise levels depicted in the top right and bottom left plots. When looking at the 50D's ISO 6,400 shots, many bright noise pixels are quite evident, especially in darker areas.

Here's the same set of noise data at ISO 12,800. Again, look at the dramatic increase in noise levels in the top right and bottom left plots.

This chart compares the Canon 50D's noise performance over a range of ISOs against that of other cameras. 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 being much more important. Here, we can see that the Canon 50D's luminance noise magnitude starts out very low compared to the competition, and increases at a fairly constant rate as ISO climes to 800, where it ends up close to the level of the 40D, and slightly higher than most of its competition. Default noise reduction becomes more aggressive at ISO 1,600, resulting in slightly lower magnitude than at 800, but then rises again to levels just below the 40D at ISO 3,200. At ISO 6,400, the magnitude is higher than the Nikon D300 and Sony A700, but much lower than the Pentax K20D. The Canon 50D is the only one in the group to offer ISO 12,800, where luminance noise magnitude is quite high indeed (about the same as the K20D at ISO 6,400). Do keep in mind these measurements are taken with each camera set at default settings, so the shape or position of the curve could be influenced by the settings you choose to use. The Pentax K20D's plot is a good example of this. Its noise magnitude is higher than the others, but that's partly because the K20D's default contrast and sharpness settings are a bit on the high side.

Canon 50D 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 50D with a nominally-exposed density step target (Stouffer 4110), and the 50D's Contrast (0), Auto Lighting Optimization (Standard) and Highlight Tone Priority (Off) settings at their default positions.

Here, we can see that he tone curve shows excellent gradation in highlights, but the shadow end trails off more abruptly. Still, these are pretty good numbers compared to other current-model DSLRs. As you will see from the next plot, though, we got better results when we turned the 50D's Auto Lighting Optimizer "Off", ironically.

Here, we see the results with the 50D's Auto Light Optimization feature turned "Off". (In case you're wondering, the "Low" and "High" ALO settings did not yield better results.) The tone curve doesn't end quite as abruptly on the shadow end as it did with ALO set to "Standard". Dynamic range at the high quality level increased slightly from 7.68 to 7.83 EV, while total dynamic range also rose slightly from 9.89 to 10.2 EV.

Processing the Canon 50D's RAW (.CR2) files through Adobe Camera Raw (ACR) version 4.6 increased the dynamic range score by about 1/3 EV at the highest quality level, compared to the in-camera JPEG with Auto Light Optimization (ALO) set to Standard (the default), but only by about 1/4 EV compared to the JPEG with no ALO applied. Total dynamic range increased more dramatically from 10.2 to 12.7 EV. These results were obtained by using ACR's automatic settings, then tweaking the sliders manually. It's also worth noting here that ACR's default noise reduction settings further reduced the chroma noise somewhat in lighter areas (see the plot in the lower left-hand corner) relative to the levels in the in-camera JPEG. But because the levels in the darker steps of the Stouffer chart were boosted to make the gamma curve trail-off more gradually, shadow noise was amplified as well, limiting the Imatest DR scores at higher quality levels.

Dynamic Range, the bottom line:

The net result was that the 50D performed well when compared against most current DSLR models, positioned near the top of the current crop of such cameras for in-camera JPEG dynamic range results. Results from processed RAW files were good, but lag behind much of the competition.

To get some perspective, here's a summary of the Canon 50D'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.):

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 DSLRs, 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 deserved its place atop the list, as its unique "SR" technology did indeed deliver a very obvious improvement in tonal range in the highlight portion of the tonal scale relative to competing models of its day. (Amazing that it's now surpassed by even consumer-level models using today's technology.) I was also surprised to see the analytical results place the original Olympus E-300 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 DSLRs 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 base 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 (like the D40) at a disadvantage, as their lowest ISO setting is 200, as compared to the ISO 100 settings available on most other models.

Canon 50D 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 we focus on here. The uncorrected resolution figures are 2,095 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 2,070 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 2,082 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased vertical resolution by quite a bit, but horizontal resolution increased only slightly, resulting in an average of 2,315 LW/PH.

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 a nearly ideal amount of in-camera sharpening is applied in the horizontal direction (undersharpened by only 4.22%, explaining why standardized sharpening wasn't able to improve on the MTF 50 numbers much), while in-camera sharpening is not as aggressive in the vertical direction (13 % undersharpened). Despite the "undersharpening" reported by Imatest though, there is a noticeable "bump" visible in the horizontal edge profile. (A very slight one in the vertical edge profile, but probably negligible.) You should thus turn the camera's sharpening down a little for optimal results when sharpening in-camera JPEGs post-exposure in Adobe Photoshop or other image editing software.