Olympus E-520 Imatest Results

Olympus E-520 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 Olympus E-520:

The Olympus E-520 showed pretty good color accuracy, as well as fairly accurate saturation levels. Hue accuracy was quite good, with most of the hue shift occurring in the cyans, blues and yellows through oranges. Average saturation was 107.8% (7.8% oversaturated) and average "delta-C" color error was only 5.48 after correction for saturation, which is also 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.) Overall, a very good color response for an SLR, especially a consumer oriented model. 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 by most computer monitors), the Olympus E-520 delivers more highly saturated color, with an average saturation of 113.4% and average saturation-corrected hue error of 5.48 "delta-C" units. Again, mouse over the links below the illustration above to compare results with other recent SLRs.

This image shows how the Olympus E-520 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 ideal 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 ideal color, without any luminance correction. This image shows the good hue accuracy, as well as a contrast curve that results in slight overexposure of the lighter swatches, and a lightening of some colors, particularly shades of blue. Overall though, the colors are quite close to their "correct" values.

There's a lot in this particular graph; as always, 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 the Canon EOS-1Ds Mark II, which produced remarkably fine-grained image noise, even at very high ISOs.

While overall noise levels are a bit higher than most competing models, the Olympus E-520 does a very good job of keeping plenty of the noise energy at high spatial frequencies and is in fact better than most cameras in this respect, at least at lower ISO settings. Overall, noise looks to be a little higher than that of the E-420, but what little low-ISO image noise that's there is fine-grained. The red channel is noisier than the others, but that's relatively common among cameras we've tested recently , and only slightly visible when inspecting low ISO shots very closely.

Here's the same set of noise data at ISO 1,600. Here, the Noise Spectrum graph is shifted quite a bit more toward the left-hand, lower-frequency side than it was at ISO 100 (note how the whole graph is piled up on the left side of the plot), coarsening the "grain" of the image noise patterns quite a bit. However, this is an improvement over the E-510, which had a distribution shifted even more to the left. Compared to the E-420, high-ISO noise characteristics are very similar, if a touch higher.

This chart compares the Olympus E-520'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 Olympus E-520's luminance noise magnitude echoes that of the E-420, though at slightly higher levels. 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 K200D's plot is a good example of this. Its noise magnitude is higher than the others, but that's partly because the K200D's default contrast and sharpness settings are a bit on the high side, while its default noise reduction is "off" (most cameras default to a medium high ISO NR setting).

Olympus E-520 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 Olympus E-520 with a nominally-exposed density step target (Stouffer 4110), and the E-520's settings at their default positions.

While better than the E-510, these are still disappointing numbers for a current-model DSLR, and reflect the tendency for the E-520's contrasty tone curve to plug deep shadows, as well as a tendency toward slightly high noise. This is somewhat apparent in the tone curve above, revealed by the way the shadow end of the curve tails off with larger spacing between the steps, and odd "tail" as it approaches black. This behavior of the E-520 was also observed by the testers and reviewers, who complained that the E-520's images sometimes took a lot of tweaking to compensate for the overly-contrasty tone curve.

Processing the E-520's RAW (ORF) files through Adobe Camera Raw (ACR) version 4.5 improved dynamic range by more than a full stop at the highest quality level. Some of the gain is likely due to ACR's more effective default noise reduction, which tends to shift dynamic range values towards the higher quality levels. (It bears noting though, that extreme manipulation of RAW files to extend dynamic range can introduce severe color shifts in the resulting files: This dramatically increased dynamic range may only be really usable when converting the images to black and white. - As is the case with most cameras.) These results were obtained by using ACR's automatic settings; slightly better results may be possible by adjusting the sliders manually.

Dynamic Range, the bottom line:

The net result was that the E-520's dynamic range is rather poor, when compared against most current DSLR models, positioning it at the bottom of the current crop of DSLRs. The E-520's slightly higher noise levels placed it below its sibling, the E-420. We've often wondered why this is, as we believe both models use the same sensor. Noise in electronics is however quite sensitive to factors such as component placement, PCB characteristics, trace routing, power distribution and grounding, decoupling, etc., so we speculate that the local differences in design required to accommodate the addition of the CCD-shift mechanism may be to blame. Or, perhaps it's just plain sample variation. Whatever the case, the E-520's slightly higher noise is reflected in Imatest's noise and dynamic range analysis.

To get some perspective, here's a summary of the Olympus E-520'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 d-SLRs, was that here again, purely analytical measurements don't necessarily correlate precisely with actual photographic experience: Some cameras with excellent shadow detail (the Nikon D2x from a couple of years back, for example) didn't place as highly as we thought they should have, given their real-world performance. On the other hand, its clear that cameras like the EOS-1Ds Mark III deserve a place high in the listings, and that cameras like the Olympus E-510 lack the dynamic range of many competitors, but the specific relative placements between some models may not correspond directly with real-world experience. - So use these numbers as general guidelines, and examine the files of cameras you're interested in visually in Photoshop or another image-editing package, to see how they stack up in terms of real-world behavior.

One thing to also note here, 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 (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.

Olympus E-520 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 1,728 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,590 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,659 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased both vertical and horizontal resolution slightly, resulting in an average of 1,940 LW/PH, noticeably lower than the best we've seen for a 10-megapixel camera.

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 there is some in-camera sharpening applied (the bump at the top ends of the black curves), that Imatest's standard sharpening operator increased slightly.