Pentax K-x Imatest Results

Pentax K-x 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-x:

The Pentax K-x showed slightly lower than average color accuracy, and its default saturation is quite a bit higher than that of the competition. Average saturation was 121.4% (21.4% oversaturated), much higher than average for an SLR (the rest in this group are in the 4-5% oversaturation range). This is quite typical for a Pentax SLR, as the camera defaults to "Bright" Image Tone mode. Hue accuracy was good, but not great, with an average "delta-C" color error of 7.14 after correction for saturation, one of the lower SLR scores in recent memory. (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 very vibrant) default color response for a digital SLR. Mouse over the links below the illustration above to compare results with other consumer 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-x delivers even more highly saturated color than sRGB, with an average saturation of 129.2%. Average saturation-corrected hue error was 7.11 "delta-C" units, only slightly better than its sRGB results. Again, mouse over the links below the illustration above to compare results with other consumer SLRs.

This image shows how the Pentax K-x 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 slightly below-average hue accuracy and 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.

The bottom row of the chart shows exaggerated white balance errors, revealing that the camera produced a slightly cool color balance when using Custom white balance, with a WhiBal card as the reference. White balance is actually pretty close for midtone through highlight values (note that the examples above deliberately exaggerate the white balance error). It's somewhat worse in the shadows, but is also less visually apparent there.

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-x does a pretty good 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 chrominance noise is only slightly visible when inspecting dark midtones and shadow areas of low ISO shots very closely.

Looking at the lower-left and upper-right plots in the illustration above, the Pentax K-x's overall noise levels are lower than those of the K-7 at ISO 100. That's not surprising, since the Pentax K-7 has smaller photosites.

Above is the same plot for ISO 200, the Pentax K-x's default base ISO. Noise levels are a little higher than ISO 100, as you'd expect, but the noise spectrum plot is very similar. This translates to slightly higher noise, mainly in the shadows, but noise levels are still quite low.

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 200, though it's hard to tell because of the change in scale. 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-x's ISO 3,200 images, though there still a lot of fine detail left intact. The red, green, blue and luminance channels are almost perfectly in-line each other, indicating the Pentax K-x's default high ISO noise reduction is doing a very good job at keeping chrominance noise in check. Also note the channels in the noise level plots in the lower left and upper right track very closely as well.

Here's the same set of noise data at ISO 6,400. The Noise Spectrum graph is again quite similar, but notice the noise levels are much higher.

Here's the same set of noise data at ISO 12,800. Here you can see a dramatic shift to the left in the Noise Spectrum graph.

This chart compares the Pentax K-x's noise performance over a range of ISOs against that of other current consumer 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-x's luminance noise magnitude starts out about average relative to its APS-C sensor "competition," and stays about average or slightly below average up to ISO 800. At ISO 1,600, the K-x's noise magnitude is about the same as the Nikon D5000, lower than the Canon XSi and Olympus E-620, but higher than the Sony A330. At ISO 3,200, it's a bit higher the the D5000 and A330, but still much lower than the Olympus E-620. The K-x remains a littler higher than the D5000 at ISO 6,400, and is the only camera in the group to offer ISO 12,800. Overall, the K-x's graph is fairly linear, indicating a nice, progressive approach to noise reduction levels versus ISO sensitivity. 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. (The Pentax K-x offers Off, Low, Medium and High noise reduction options, plus a programmable start ISO ranging from ISO 400 to 3,200 in 1 EV steps. The default settings are Medium NR starting at ISO 800.)

Pentax K-x 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-x with a nominally-exposed density step target (Stouffer 4110), and the K-x's settings such as Contrast and D-Range settings at their default positions. We used ISO 100 for the K-x, as Imatest dynamic range results were slightly better than ISO 200.

Here, we can see that the tone curve shows pretty good gradation in the highlights and the shadows trail off nicely too. Total dynamic range is good (at 10.6 f-stops), however the score at the highest level (7.18) is a little on the low side for a modern APS-C sensor SLR, likely in-part due to the K-x's slightly high default contrast and sharpening, which tends to amplify noise.

Processing the Pentax K-x's RAW (.DNG) files through Adobe Camera Raw (ACR) version 5.6 beta increased dynamic range by more than two full f-stops at the highest quality level (9.33) compared to the in-camera JPEG (7.18), while increasing total dynamic range from 10.6 to 11.4 f-stops. 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. Also, the extreme highlight recovery being performed by ACR here would likely produce color errors in strong highlights of natural subjects.

Dynamic Range, the bottom line:

The net result was that the Pentax K-x's JPEGs scored a bit below average in Imatest's dynamic range analysis when compared against other current APS-C sensor models. Pentax K-x RAW files converted with ACR did much better, placing the K-x in the top five cameras tested to date. When it comes to dynamic range (and high ISO noise), the K-x did much better than its more expensive sibling, the Pentax K-7.

To get some perspective, here's a summary of the Pentax K-x'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-x, as noted above, its somewhat low scores for camera JPEGs reflect Pentax' decision use slightly high contrast and sharpening at its default settings. You can see noise is more visible in the K-x's JPEGs than in dcraw-converted RAW files (look for JPEG files with the suffix _DCR), so sensor noise levels are not the issue here.

Pentax K-x 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 1,371 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,477 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of only 1,424 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 1,992 LW/PH. The corrected numbers are slightly below what we'd expect from a 12-megapixel DSLR, though not too far off the mark.

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 20.4 %), as well as in the vertical direction (18% undersharpened), explaining why standardized sharpening increased the MTF 50 numbers by a fair amount.

As with the K-7, 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-x's images by careful processing of its RAW files.