Pentax K-5 Imatest Results

Pentax K-5 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-5:

Like all recent Pentax SLRs, the K-5 showed decent color accuracy, though its average default saturation is quite a bit higher than that of its competitors. Hue accuracy was acceptable, with small to moderate hue shifts occurring in many colors. Average saturation was 124.5% (24.5% oversaturated) which is the highest default saturation we've seen in a while from an SLR. Average "delta-C" color error was 6.11 after correction for saturation, which while not terrible, is the poorest performance of the group above. (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 fair (and vibrant) default color response for a prosumer digital SLR. Mouse over the links below the illustration above to compare results with other prosumer models.

Most SLRs oversaturate colors when shooting in Adobe RGB mode, and the Pentax K-5 is no exception. Average saturation was a whooping 130.4% which is higher than the sRGB result. Average saturation-corrected hue error was 5.73 "delta-C" units, slightly better accuracy than sRGB. A pretty typical result for a Pentax at default settings. Mouse over the links below the illustration above to compare results with the same group.

This image shows how the Pentax K-5 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 decent hue accuracy and somewhat 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 somewhat cool color balance when using Custom white balance, with a WhiBal card as the reference. (Note that the examples above deliberately exaggerate the white balance error.)

As always, there's more in this particular graph than we really have room to go into here. (Also note that 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 often 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.

Above, we see the results at ISO 80. The luminance noise power plot (lower-right, in black) is quite flat indicating a very fine noise grain with plenty of energy at higher frequencies. The chroma channel noise levels are significantly elevated, especially red and blue. The dominance of low frequencies in the color channels points to minor blotchiness that can be seen in some midtones and shadows when viewing at 100%.

Above, we see the results at ISO 100. Results are very similar to ISO 80, with just slightly higher noise levels. Still, noise levels are much improved over the Pentax K-7 at ISO 100, but are a little higher than those from other state-of-the-art APS-C models such as the Nikon D7000 and Canon 60D.

Above is the same set of noise data from the Pentax K-5 at ISO 3,200. Here, the overall Noise Spectrum graph is shifted a fair bit toward the left-hand, lower-frequency side than it was at ISO 100, coarsening the "grain" of the image noise patterns. There's not much difference between the color channels in this plot, with all color channels tracking luminance noise very closely, no doubt due to clever noise reduction processing. Indeed, high ISO JPEGs from the Pentax K-5 show very low levels of chroma noise. Overall noise levels are much lower than the Pentax K-7, slightly lower than the Canon 60D (but significantly lower in the shadows), and similar to the Nikon D7000. An excellent performance.

Here's the same set of noise data at ISO 6,400. Here again, the Noise Spectrum graph is shifted slightly more toward the left-hand side, further coarsening the "grain" of the image noise patterns. Overall noise levels are higher than ISO 3,200 as you'd expect, but noise performance (particularly chroma noise), is equal to or better than the best APS-C SLRs of similar resolution we've tested to date.

This above chart is a little crowded, so below is a version with ISO range 100-6,400.

This chart compares the Pentax K-5's noise performance over a range of ISOs against that of a few other SLR cameras in a similar price range or resolution. 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, not to mention what the camera does to achieve a given noise level. (Some cameras obliterate subject detail along with noise, so the camera with the lowest noise levels on MacBeth chart swatches may not necessarily produce the most appealing images.)

As you can see, the Pentax K-5's luminance noise levels are slightly higher than most of the cameras in this group up to ISO 1,600 where noise reduction kicks in. From then, noise levels are competitive. However, noise levels are significantly lower than the K-7 across the board, and the K-5's noise reduction is generally quite adept at leaving plenty of detail behind even at higher ISOs. Default noise reduction does however struggle a bit with subtle tone-on-tone detail such as in the red leaf cloth swatch in our Still Life shots, where it performs similar to the K-7. Still, an excellent performance from the Pentax K-5 overall. Keep in mind, though, that these results are at default noise reduction settings, so the shape and positions of the curves will be influenced by your settings.

 

Pentax K-5 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-5 with a nominally-exposed density step target (Stouffer 4110), with its settings such as contrast and D-range set to the defaults.

Here, we can see that the tone curve trails off fairly gradually at both the highlight and shadow ends, but default contrast is a bit high. Total dynamic range is good at 10.5 f-stops, though about the same the K-7's 10.7 f-stops, so the camera isn't making as much of the sensor's improved dynamic range as it could. (No surprise, when using the rather contrasty default "Bright" setting.) The high quality score is a little low compared to recent APS-C sensors, a result of the slightly higher noise levels at low ISOs when using the default settings. (ISO 80 yielded the best scores for the Pentax K-5.)

The illustration above shows the results from an Adobe Camera Raw 6.3 converted DNG file, using the Auto setting. Slightly better results are possible by manually tweaking, but we weren't able to improve the scores. As can be seen, the score at the highest quality level was much better than the in-camera JPEG, at 10.2 versus 7.22 f-stops, which is the highest score to date of any camera we've tested. Total dynamic range also improved a full stop, from 10.5 to 11.5 f-stops. As is often the case, it's worth noting here that ACR's noise reduction reduced overall noise (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-5's JPEGs using default settings showed slightly disappointing dynamic range, likely due to the high default contrast and saturation, as well as slightly higher than average noise levels. RAW dynamic range performance with Adobe Camera RAW was however outstanding, producing the best high quality score we've tested to date, besting even full-frame models. Dynamic range at lower quality levels was very good at 11.5 f-stops, but not as good as some competitors.

To get some perspective, here's a summary of the Pentax K-5'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.):

Note that this test is repeatable to within 1/3 EV according to the Imatest website, so differences of less than 0.33 can be ignored.

Pentax K-5 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 1,731 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,695 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of only 1,713 LW/PH. That's lower than you'd expect from a 16-megapixel camera. Correcting to a "standardized" sharpening with a one-pixel radius improved the numbers significantly, however, resulting in an average of 2,336 LW/PH, bringing the K-5 in-line with other models of similar resolution. The much improved standardized versus uncorrected results indicate the Pentax K-5 is applying a conservative amount of in-camera sharpening to its JPEGs at default settings.

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 modest level of in-camera sharpening is applied in both the horizontal and vertical directions. Imatest reports 16.1% undersharpening in the horizontal direction, and 16.2 % undersharpening in the vertical direction. This means that the Pentax K-5's JPEG images will respond well to some additional post-exposure sharpening in Adobe Photoshop or other image editing software, without too many visible sharpening artifacts. (Of course for best results, shoot RAW and apply all sharpening while post-processing.)

Note: We don't feature SFR-based LW/PH resolution numbers more prominently in our reviews (eg, outside the Imatest pages) because we've found that they're *very* sensitive to minor differences in in-camera image processing. Relatively small changes in the amount of in-camera sharpening can have a large effect on the resulting resolution numbers. Imatest attempts to compensate for this by adjusting to a "standard" sharpening, but this approach can't completely undo what happens inside the cameras, and so often gives inconsistent results. Sometimes the "standardized" sharpening happens to just match the shape of the edge profile with the in-camera sharpening applied, and you'll get wildly high results. At other times, it will tend to correct in the opposite direction. Unfortunately, ignoring the in-camera sharpening entirely can result in even greater discrepancies, particularly between models from different manufacturers. Turning off sharpening in the camera may or may not fully eliminate the sharpening, so simply turning off sharpening in the camera JPEGs isn't a reliable solution. It also wouldn't be the way most people shoot the cameras. We could process RAW files with no sharpening (as shown below), but then that'd only suit the people working primarily or exclusively from RAW, and would open another can of worms as to what RAW converter was used, etc, etc.

The bottom line is that numbers for resolution only take you so far. Detail handling and edge acuity are very complex issues; ones that don't easily boil down to a single number. The best approach is to simply look look at the broad array of standardized test shots we take with each camera, to the point of downloading and printing them with whatever processing you'd use if you owned the camera and shot with it. See how the differences stack up for you visually, and make your decision on that, rather than on abstract resolution numbers.