Olympus E-420 Imatest Results

Olympus E-420 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-420:

The Olympus E-420 showed very 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 107.3% (7.3% oversaturated) and average "delta-C" color error was only 5.04 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), the Olympus E-420 delivers more highly saturated color, with an average saturation of 113.9% and average saturation-corrected hue error of 5.00 "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-420 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 very good hue accuracy, as well as a contrast curve that results in slight overexposure of the darker swatches and slight underexposure of lighter ones. Overall though, the colors are impressively 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-420 does a very good job of keeping plenty of the noise energy at high frequencies and is in fact better than most cameras in this respect, at least at lower ISO settings. What little low-ISO image noise that's there is more fine-grained as a result. The red and especially blue channels are a little noisier than the others, but that's not at all unusual , and not visible even when closely inspecting low ISO shots.

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-410, which had a distribution shifted even more to the left. In the E-420 though, the overall noise amplitude remains quite low, lower than that of most of the competition. The overall effect is actually quite pleasing: ISO 1,600 images are quite detailed with a tight grain structure which for the most part manages to not appear blotchy. (There is more blotchiness in the red channel, which may appear in some images but not others, depending on the colors of particular objects you might be looking at.) A good job, and improvement on the earlier E-410.

This chart compares the Olympus E-420'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. Here, we can see that the Olympus E-420's luminance noise starts out a bit high compared to the competition, reduces a bit at ISO 200, then increases at roughly the same rate as the others to ISO 400. It doesn't rise much at ISO 800, where where it is lower than the others. Then we see E-420 noise levels increase again at ISO 1600, although still remaining at or below most of the competition (with the notable exception of the Sony A200, which is applies more aggressive noise reduction). 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.

Olympus E-420 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-420 with a nominally-exposed density step target (Stouffer 4110), and the E-420's contrast and DRO settings at their default positions.

While better than the E-410, these are still disappointing numbers for a current-model DSLR, and reflect the tendency for the E420's contrasty tone curve to plug deep shadows pretty badly. This is somewhat apparent in the tone curve above, revealed by the way the shadow end of the curve tails off with large spacing between the steps, and odd "tail" as it approaches black. This behavior of the E-420 was also observed by the testers and reviewers, who complained that the E-420's images sometimes took a lot of tweaking to compensate for the overly-contrasty tone curve.

Processing the E-420's RAW (ORF) files through Adobe Camera Raw (ACR) version 4.4.1 improved dynamic range by more than a full stop at the highest quality level. (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. (Note: ACR version 4.4.1 supported the E-410 but not the E-420. We edited the file header to trick ACR 4.4.1 into processing the E-420's files as if they came from the E-410, which worked quite well.)

Dynamic Range, the bottom line:

The net result was that the E-420 performed rather poorly when compared against most current DSLR models, positioning it near the bottom of the current crop of DSLRs.

To get some perspective, here's a summary of the Olympus E-420'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 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 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 (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-420 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 1,980 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,836 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,908 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased both vertical and horizontal resolution slightly, resulting in an average of 2,028 LW/PH, just a bit below 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's a significant amount of in-camera sharpening applied (the large bump at the top ends of the black curves), that the standard sharpening operator reduces slightly. (Imatest complained that we were clipping the shadows a bit here (which would actually tend to improve the apparent edge sharpness somewhat), but that's just a consequence of the E-420's high contrast. We couldn't eliminate the shadow clipping without clipping the highlights instead.)