Pentax K-7 Imatest Results

Pentax K-7 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 Pentax K-7:

Like it's sibling, the Pentax K20D, the K-7 showed fairly good color accuracy, though its average default saturation is quite a bit higher than that of most competitors. Hue accuracy was good, with relatively small hue shifts occurring in many colors. Average saturation was 118.9% (18.9% oversaturated) and average "delta-C" color error was 6.04 after correction for saturation, which while not bad, isn't quite as good as some of the competition. (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 good (albeit vibrant) default color response for a prosumer digital SLR. Mouse over the links below the illustration above to compare results with other prosumer 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 Pentax K-7 delivers more highly saturated color. Average saturation was 126.4%, and average saturation-corrected hue error was 5.98 "delta-C" units, actually slightly more accurate than the sRGB result. Again, mouse over the links below the illustration above to compare results with other prosumer SLRs.

This image shows how the Pentax K-7 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 generally good hue accuracy and slightly high saturation of most colors. It also shows the tendency towards a greenish tinge in the range of yellow through orange, as well as the significant saturation boost in greens and blues.

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 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.

Except for the red channel, the Pentax K-7 does an excellent job of keeping plenty of the noise energy at high frequencies. What little low-ISO image noise that's there is more fine-grained as a result. The red channel has a lot more low-frequency energy, but that seems to be fairly common these days, and is only slightly visible when inspecting shadow areas of low ISO shots very closely.

Looking at the lower-left plot in the illustration above, the K-7's overall noise levels are similar to those the K20D, just slightly higher, except in the shadows where the K-7's noise is significantly higher, both in the luminance channel and in the RGB channels.

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, though it's hard to tell because of the change in scale. (Interestingly, the red channel's noise spectrum now closely tracks the others, vs its behavior at low ISOs.) 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 K-7's ISO 3,200 images, though there still a lot of fine detail left intact. The red, green and blue channels are more in-line with the others, most likely the result of Pentax's default high ISO noise reduction. Looking at the lower left graph again, noise is higher than the K20D in the deepest shadows, but a bit lower at higher brightnesses.

Here's the same set of noise data at ISO 6,400. The Noise Spectrum graph is similar though more linear, but notice the noise levels are much higher. (Particularly evident in the pixel noise plot in the upper right corner of the illustration, and in the midtones and below in the graph in the lower left hand corner.)

This chart compares the Pentax K-7's noise performance over a range of ISOs against that of other current prosumer models. 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 Pentax K-7's luminance noise magnitude starts out a little higher than average relative to its "competition", and stays higher than the rest up to ISO 1,600, where it's about equal to the K20D's. At ISOs 3,200 and 6,400, K-7 luminance noise levels are lower than its sibling, but much higher than the others in this group. It's interesting to note that while the Pentax K20D's High ISONR setting defaults to Off, the K-7 defaults to Medium, implying the K-7's sensor is noisier. Indeed, our dcraw converted RAW files confirm that. Do keep in mind these measurements are at default settings, so the shape or position of the curve is influenced by the settings you choose to use. Selecting a higher noise reduction setting (the Pentax K-7 has Off, Low, Medium and High options) would have certainly lowered the values in the graph above, but at the expense of fine detail.

Pentax K-7 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 Pentax K-7 with a nominally-exposed density step target (Stouffer 4110), and the K-7's settings such as Contrast and D-Range settings at their default positions.

Here, we can see that he tone curve shows pretty good gradation in the highlights and the shadows trail off nicely too. Total dynamic range is good (at 10.7 f-stops), however the score (6.54) at the highest level is a little on the low side for a modern APS-C sensor, likely due to noise in darker tones. Indeed, when deep shadows are closely observed in K-7 images, a moderate amount of noise is visible, though there is also a lot of detail present. We find this far preferable to low noise with detail obliterated by over-aggressive noise reduction.

Processing the Pentax K-7's RAW (.DNG) files through Adobe Camera Raw (ACR) version 5.4 increased dynamic range by more than a full f-stop at the highest quality level (7.73), compared to the in-camera JPEG (6.54). These results were obtained by using ACR's automatic settings, and we weren't able to do any better with manual adjustment. It's worth noting here is that ACR's default noise reduction settings reduced overall noise somewhat (see the plot in the lower left-hand corner) relative to the levels in the in-camera JPEG, which would tend to boost the dynamic range numbers for the High Quality threshold.

Dynamic Range, the bottom line:

The net result was that the Pentax K-7's JPEGs scored a bit below average in Imatest's dynamic range analysis when compared against other current APS-C sensor models. Pentax K-7 RAW files converted with ACR did better, but results were still below average for its class, and below the K20D's scores.

To get some perspective, here's a summary of the Pentax K-7'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 (Dave) 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 once 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 test 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.

When it comes to the Pentax K-7, as noted above, its low scores for camera JPEGs reflect Pentax' decision to leave more image noise in its files at default settings, so as to not have to trade away as much subject detail. There's quite a bit of subject detail visible in the shadows of the K-7's images, but its dynamic range rankings in Imatest are brought down by the noise that's also present. We're finding that we personally prefer this approach to cameras that smudge away subtle subject detail just to achieve lower noise levels in areas of flat tint.

Pentax K-7 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 1,606 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,486 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of only 1,546 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased the resolution score by quite a bit, resulting in a much higher average of 2,545 LW/PH. The corrected numbers put the K-7 pretty much on par with its competitors.

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 very little if any in-camera sharpening is applied in the horizontal direction (undersharpened by 20.6 %), as well as in the vertical direction (22.8 % undersharpened), explaining why standardized sharpening increased the MTF 50 numbers so much.

As with the K20D, we found these results a little paradoxical, given that we definitely saw sharpening artifacts in most of our "real world" images. (The far-field shot and the Still Life, in particular.) In-camera sharpening appears to vary depending on the density level of structures in its images. At very high and low densities (the black and white areas on the resolution target), it applies almost no sharpening at all. For densities away from the extremes of the tonal scale (the branches against the sky in our far-field shot and some of the details on the bottle labels in the Still Life shot), more sharpening is applied, resulting in slight halos around contrasting objects.

Overall (as is almost always the case), you'll extract the most detail from the Pentax K-7's images by careful processing of its DNG RAW files.