Pentax K-x Image Quality

Saturation & Hue Accuracy
Bright, intense colors with moderate oversaturation of strong blues, reds, and greens.

Saturation. Like all recent Pentax DSLRs, the K-x's default settings push most colors by quite a bit, especially blues, greens, and some reds. Overall, images were very bright and punchy, with color that was a little over the top for an SLR, though the K-x's target market would likely prefer that to more accurate saturation. You can, however, always select a different Image Tone setting or tweak saturation and contrast to suit your own tastes. Most consumer digital cameras produce color that's more highly saturated (more intense) than found in the original subjects. This is simply because most people like their color a bit brighter than life.

Skin tones. Caucasian skin tones from the K-x lean toward the warm side, but many consumers prefer the "healthier" appearance of warmer skin tones. Where oversaturation is most problematic is on Caucasian skin tones, as it's very easy for these "memory colors" to be seen as too bright, too pink, too yellow, etc.

Hue. The Pentax K-x showed a number of color shifts relative to the correct mathematical translation of colors in its subjects, but had acceptable accuracy overall. Most noticeable was a shift in orange toward yellow, yellow toward green, with some shifts in cyans, blues, and reds as well. Hue is "what color" the color is.

Saturation Adjustment
The Pentax K-x has a total of nine saturation settings available, four above and four below the default saturation. This covers a pretty wide range of saturation levels, about as wide a range as you're likely to find photographically relevant, apart from special effects that are arguably better achieved in software. The fine steps between settings mean you can program the camera to just the level of saturation you prefer. The saturation adjustment also has almost no impact on contrast. That's how it should work, but we've often found interactions between saturation and contrast (and vice versa) on the cameras we test.

The table above shows results with several saturation settings. Click on any thumbnail above to see the full-sized image.

Exposure and White Balance

Indoors, incandescent lighting
Warm results with default Auto, slightly cool with Incandescent, though good color with Manual white balance setting. Average exposure compensation required.

Indoors, under normal incandescent lighting, color balance was quite warm with the default Auto white balance setting, with a fairly strong orange cast; however, this is considerably better than Pentax SLRs used to produce which was a bright yellow/orange cast. Like the K-7, Pentax K-x features an "Automatic White Balance in Tungsten" setting in Custom menu 9. Options are "Subtle" and "Strong" correction, with the default being Subtle. The Strong setting resulted in a reduced orange cast compared to Subtle, with a slightly magenta tint. Results with the Incandescent setting were a bit on the cool side, with a slightly bluish cast. The Manual setting was the most neutral and accurate. Note that there is also a Color Temperature Enhancement (CTE) white balance mode, which strongly intensified the orange/yellow cast in our indoor test shot. The Pentax K-x required an average amount of positive exposure compensation for our indoor scene, at +0.3 EV. Overall color with the Manual white balance setting looks quite good, though the blue flowers appear slightly purple. (Many digital cameras reproduce these flowers with a dark, purplish tint, so the Pentax K-x actually performs a little better than average here.) Our test lighting for this shot is a mixture of 60 and 100 watt household incandescent bulbs, a pretty yellow light source (2,600 Kelvin), but a very common one in typical home settings here in the U.S.

Outdoors, daylight
Bright colors overall, though a tendency toward a warm cast and slightly high contrast under harsh lighting.

Outdoors, the Pentax K-x performed fairly well, requiring about average exposure compensation of +0.7 EV for our "sunlit" portrait shot to keep the face reasonably bright. Contrast is a little high, as you might expect under such harsh lighting, and highlights in the model's shirt and flowers are clipped, but there's plenty of detail in the deep shadows. Hardly any highlights were blown out in the House shot, and shadows were pretty good, too. Color balance is good as well, if a bit warm, though default saturation is a little high. Overall, very good performance here.

See full set of test images with explanations
See thumbnails of all test and gallery images

Resolution
High resolution, 1,600 ~ 1,700 lines of strong detail.

Our laboratory resolution chart revealed sharp, distinct line patterns down to about 1,700 lines per picture height in the horizontal direction, and about 1,600 in the vertical direction. Extinction didn't occur until around 2,400 to 2,600 lines. We weren't able to do much better with Adobe Camera RAW processed RAW files, though results were crisper. There were quite a few moire interference patterns near or above the resolution limit in both the camera JPEG and ACR converted RAW file, indicating a fairly weak anti-alias (AA) filter (see other areas of the resolution target). Use these numbers to compare with other cameras of similar resolution, or use them to see just what higher resolution can mean in terms of potential detail.

See full set of test images with explanations
See thumbnails of all test and gallery images

Sharpness & Detail
Sharp images overall, though visible edge-enhancement on high-contrast subjects. Minimal noise suppression visible.

Sharpness. The Pentax K-x produced sharp images with good detail. Some edge enhancement artifacts in the form of "halos" are visible on high-contrast subjects such as the crop above left, but overall results are still pretty good. Edge enhancement creates the illusion of sharpness by enhancing colors and tones right at the edge of a rapid transition in color or tone.

Detail. The crop above right shows only minimal noise suppression, as the darker areas of the model's hair show a lot of detail. Individual strands are still distinguishable even in the lighter shadows, though they begin to merge as shadows deepens. Still, a great performance, as many SLRs have quite a bit more trouble with the straight, red strands of hair here. Noise-suppression systems in digital cameras tend to flatten-out detail in areas of subtle contrast. The effects can often be seen in shots of human hair, where the individual strands are lost and an almost "watercolor" look appears.

RAW vs In-Camera JPEGs
As noted above, the Pentax K-x delivers sharp images, but with some visible sharpening artifacts in camera JPEGs. (At least in this shot - the camera appears to adapt sharpening depending on the image structure.) More detail can be obtained from carefully processing RAW files, though, while at the same time reducing sharpening artifacts. Take a look below, to see what we mean:

In the table above, mousing over a link at the bottom will load the corresponding crop in the area above. Examples include in-camera Fine (***) JPEG, and RAW (.DNG) file processed through Adobe Camera Raw version 5.6b, then sharpened in Photoshop. For the Pentax K-x's images, I found best results with strong but tight 300% unsharp masking with an 0.3 pixel radius. As you can see, the JPEG has quite a bit more contrast, but the ACR conversion contains more detail than the JPEG, and there are fewer sharpening artifacts as well. We should mention that in addition to 9 sharpening levels, the K-x has a "Fine Sharpening" option, however we didn't test those settings.

ISO & Noise Performance
Low noise at the normal sensitivity settings, with excellent results up to ISO 400, and surprisingly good results up to ISO 1,600. 

The Pentax K-x produced low noise at its lower sensitivity settings, and even at ISO 400, noise is fairly low, with little detail lost to the noise reduction. (The K-x's default noise reduction doesn't kick in until ISO 800.) At ISO 800, noise pixels are slightly less evident than at 400, though there's a small amount of smudging visible, due to noise reduction. Still, the K-x performs better than average for its class, as there is still a lot of fine detail left. ISO 1,600 shows a bit more blurring from stronger noise reduction, but detail is still pretty good for a high ISO. ISO 3,200 isn't bad either, though there is a more noticeable drop in fine detail, and there are some greenish chroma noise blotches visible in shadow areas and in the hair. At the highest ISO settings of 6,400 and 12,800, noise levels are much higher, with heavy blurring and chroma noise blotches. We also noticed some moire interference patterns in the green jacket, at lower ISOs. Still, much better than average performance here for an APS-C sensor. Pentax has done a good job at keeping both luminance and chrominance noise low while holding on to a lot of fine detail.

The Pentax K-x offers four levels of high ISO noise reduction (Off, Low, Medium and High), as well as a programmable start ISO (400, 800, 1,600 and 3,200). That's much more flexibility in noise reduction options than a typical entry-level SLR offers. See the Print Quality section below for recommended maximum print sizes at each ISO.

A note about focus for this shot: We shoot this image at f/4, using one of three very sharp reference lenses (70mm Sigma f/2.8 macro for most cameras, 60mm f/2.8 Nikkor macro for Nikon bodies without a drive motor, and Olympus Zuiko 50mm f/2.0 for Four Thirds and Micro Four Thirds bodies). To insure that the hair detail we use for making critical judgements about camera noise processing and detail rendering is in sharp focus at the relatively wide aperture we're shooting at, the focus target at the center of the scene is on a movable stand. This lets us compensate for front- or back-focus by different camera bodies, even those that lack micro-focus adjustments. This does mean, though, that the focus target itself may appear soft or slightly out of focus for bodies that front- or back-focused with the reference lens. If you click to view the full-size image for one of these shots and notice that the focus target is fuzzy, you don't need to email and tell us about it; we already know it. :-) The focus target position will simply have been adjusted to insure that the rest of the scene is focused properly.

Extremes: Sunlit and low light tests
High resolution with strong overall detail, but slightly high contrast with strong highlights. Good low-light performance, capable of capturing bright images under average city street lighting and much darker conditions, but AF struggled at lower light levels.

Sunlight. The Pentax K-x produced slightly high contrast with some washed-out highlights and deep shadows under the harsh lighting of the test above. (The camera's default Image Tone setting is "Bright", which uses +1 out of a range of -4 to +4 for Contrast.) Shadow detail is pretty good, though, despite some minor image noise. Our outdoor target required +0.7 EV exposure compensation for a reasonably bright face, which is average for this shot. Some may prefer the +1.0 EV exposure, but we thought too many highlights were blown, and the skin tone was washed out in some places.

Because digital cameras are more like slide film than negative film (in that they tend to have a more limited tonal range), we test them in the harshest situations to see how they handle scenes with bright highlights and dark shadows, as well as what kind of sensitivity they have in low light. The shot above is designed to mimic the very harsh, contrasty effect of direct noonday sunlight, a very tough challenge for most digital cameras. (You can read details of this test here. In actual shooting conditions, be sure to use fill flash in situations like the one shown here; it's better to shoot in open shade whenever possible.)

Contrast Adjustment
We really like it when a camera gives us the ability to adjust contrast and saturation to our liking. It's even better when those adjustments cover a useful range, in steps small enough to allow for precise tweaks. Just as with its saturation adjustment, the Pentax K-x's contrast setting meets both challenges, though it could have used slightly more range on the minus side. We did, however, notice that saturation takes a hit when contrast is turned down.

At its lowest contrast setting, the Pentax K-x did an okay job of preserving highlight detail, maintaining fairly natural-looking skin tones, and holding nice detail in the shadows. There are still blown highlights, but a bit fewer than at default contrast. The Pentax K-x captures natural looking color outdoors, though just slightly on the warm side. Overall, good results here, especially when the contrast setting is tweaked, though colors are a bit flat because of the reduced saturation.

The series of shots above shows results with several different contrast adjustment settings, showing every other step, as well as the default and both extremes. While you can see the extremes, it's hard to really evaluate contrast on small thumbnails like these, click on any thumbnail to go to the full-size image. The Pentax K-x's contrast adjustment worked well, however it had quite an effect on color saturation. Contrast and saturation are actually fairly closely coupled, so this is unfortunately not unusual.

D-Range Settings
The Pentax K-x offers three Shadow Correction levels (Low, Medium, and High) as well as one Highlight Correction setting (On/Off). As the name implies, Shadow Correction works to raise shadow detail while attempting to keep highlights and midtones as they are, and likewise, Highlight Correction attempts to reduce highlights without darkening shadows and midtones. See the images and crops below to see their effect on our high contrast "Sunlit" Portrait test shot.

Above, we see a gradual lightening of shadows as the Shadow Correction setting is increased, while highlight clipping remains roughly the same. If you look closely at the shadow detail, you will notice noise is more visible as the setting is turned higher, but not as much as you might think.

In the crops above, we can see nearly all highlight detail is preserved with Highlight Correction On (right) than it is with it Off (left), while shadow levels are virtually untouched (perhaps just slightly boosted when On). Note that we used the base ISO of 200 for our "Sunlit" Portrait shots. If we had used the ISO 100 extension, we would probably see more of a difference in noise with Highlight Correction enabled. Still, an impressive performance here.

HDR Capture
The Pentax K-x has a High Dynamic Range (HDR) capture mode where the camera takes three images (underexposed, normal, and overexposed) in quick succession and combines them in-camera into one image.  If performed properly, this method should result in much higher dynamic range, without the additional noise penalty that often comes with boosting shadows. (In fact, it can reduce shadow noise by combining shadows from the overexposed shot.) There are three HDR settings available: Off (default), Standard and Strong.

As you can see from the above images, it didn't work very well for our "Sunlit" Portrait shot, resulting in very flat and unnatural looking images with reduced saturation. It did a good job of bringing out shadow detail, but even the lowest "Standard" setting was too strong for the highlights, turning white into grey. The Pentax K-x's HDR mode may work better with a different subject such as a landscape with a bright (normally washed-out) sky. It probably isn't designed to work with portraits anyway, as subject motion would likely be a problem.

Face Detection
The Pentax K-x has a face-detection AF/AE option in Live View mode.

As you can see above, it does improve the exposure over the default without using any exposure compensation, but the model's face is still a bit dim.

Low light. The Pentax K-x performed reasonably well on the low-light test, capturing usable images at the lowest light level with the lowest sensitivity setting (ISO 100). Low light exposure metering wasn't very reliable though, forcing us to use manual exposure mode. Noise was quite low up to ISO 1,600, but became more of an issue from there. Still, very good for a sub-frame SLR. There were a few hot pixels here and there, but nothing to be concerned about. (The K-x has a built-in pixel mapping function, so you should be able to map out any hot or dead pixels without sending the camera in for service.) There are hints of horizontal banding, but only at the highest ISOs, and it's not really that noticeable. Color balance was quite warm Auto white balance setting, though, showing a reddish cast at most light levels.

The camera's phase-detect autofocus system was only able to focus on the subject down to between 1/2 and 1/4 foot-candle light level unassisted with our reference Sigma f/2.8 lens. This is below average for an SLR, but the K-x was able to focus in complete darkness with the help of the AF assist from the built-in flash. Surprisingly, the Pentax K-x was able to focus in dimmer light when using the slower Pentax 18-55m f/3.5-5.6 AL II lens, down to between 1/4 and 1/8 foot-candle. This is still a bit below after for an SLR, but the K-x spent less time hunting for focus with the Pentax lens, so it found focus before giving up like it did with the Sigma f/2.8 prime.

In Live View mode, the camera was also able to focus down to between 1/4 and 1/8 foot-candle in both phase-detect and contrast-detect modes. When using phase-detect AF, it was able to focus in complete darkness with AF assist. AF assist is however not supported for contrast-detect mode, as autofocusing in that mode is too slow for the flash to assist effectively. Keep in mind that the longer shutter speeds here demand the use of a tripod to prevent any blurring from camera movement, Pentax's Anti-Shake not withstanding. (A useful trick is to just prop the camera on a convenient surface, and use its self-timer to release the shutter. This avoids any jiggling from your finger pressing the shutter button, and can work quite well when you don't have a tripod handy.)

How bright is this? The one foot-candle light level that this test begins at roughly corresponds to the brightness of typical city street-lighting at night. Cameras performing well at that level should be able to snap good-looking photos of street-lit scenes.

NOTE: This low light test is conducted with a stationary subject, and the camera mounted on a sturdy tripod. Most digital cameras will fail miserably when faced with a moving subject in dim lighting. (For example, a child's ballet recital or a holiday pageant in a gymnasium.) Digital SLRs like the Pentax K-x do much better than point & shoots, but you still shouldn't expect a quick autofocus lock with moving subjects.

Print Quality
Great print quality, good color, sharp 13 x 19-inch prints.

The Pentax K-x's printed output is really impressive, able to output usable 20 x 30-inch prints at ISO 100, straight from the camera with standard noise suppression on. They're slightly soft in some places, tack sharp in others; a little sharpening in a program like Photoshop could make them even better. 16 x 20 images are about the same, with a little more sharpness than the 20 x 30-inch prints. The amazing part is that print quality remains this good up to ISO 800! Even our troublesome red swatch still looks better than most cameras do at ISO 800, even when printed at 16 x 20.

ISO 1,600 represents the first break from this remarkable print size, and it's also the first time the red fabric swatch begins to decay. Still, the rest of the detail looks fabulous, with just a little noise and noise suppression artifacts creeping in. Reduction to 13x19 at this size is a little better.

ISO 3,200 shots also look decent printed at 13x19 inches.

ISO 6,400 shots are generally usable at 11x14, remarkably, but of course better at 8x10.

Depending on the subject ISO 12,800 shots are usable at 8x10, though contrast is increased and there's some blotchy noise in the shadows. At 5x7 inches, though, no one will notice.

A truly excellent and surprising outcome from the Pentax Kx!

Testing hundreds of digital cameras, we've found that you can only tell just so much about a camera's image quality by viewing its images on-screen. Ultimately, there's no substitute for printing a lot of images and examining them closely. For this reason, we now routinely print sample images from the cameras we test on our Canon Pro9000 Mark II studio printer, and on the Canon Pixma MP610 here in the office. (See the Canon Pixma Pro9000 Mark II review for details on that model.)