Pentax K-x Optics

Kit Lens. The Pentax K-x usually comes bundled with an smc PENTAX-DA 18-55mm F3.5-5.6 AL II kit lens. We tested this lens over at, and found it to be quite good for a kit lens, and an improvement over the previous version. Here's a link to the SLRgear review. See below for our lens test results when used with the Pentax K-x.

Lens Compatibility. The Pentax K-x features a Pentax KAF2 bayonet lens mount, and according to Pentax, is compatible with the company's entire series of K lenses. While not all functions will be available with every lens, particularly with older lenses without AF contacts, die-hard Pentax fans who already have a large collection of lenses will doubtless be pleased with the K-x's broad range of lens choices. (The camera's Custom menu offers a handful of options for lenses that cannot directly communicate with the camera, such as the use of the aperture ring and display of focus indicators.) The latest DA lenses with ultrasonic motors are fully supported; the only real limitation is that the Power Zoom function available on certain FA zoom lenses isn't supported on the K-x.

The table below (courtesy of Pentax USA), details the K-x's lens compatibility with the various types of Pentax lenses.


More points, improved function. The Pentax Kx uses an 11-point AF system vs the K2000's 5-points, and all 11 AF points are addressable, unlike the K2000 which offered only wide and center area modes. Compared to the prosumer K-7, the K-x lacks the AF point indication in the viewfinder, the dedicated AF assist lamp, and both the illuminant color correction and AF adjustment capabilities. (The grey focus points in the above image are not visible in the viewfinder.)

Pentax K-x Autofocus System. Compared to the K2000 / K-m, the new Pentax K-x offers an improved 11-point AF system that is based around the same SAFOX VIII+ sensor featured in the higher-end K-7 digital SLR, but with a few related hardware and firmware features dropped from the design.

The Pentax K-x doesn't include the light-color sensor which allows the K-7 to improve its autofocus accuracy under light sources with widely varying hues.

The Pentax K-x also lacks the dedicated AF assist lamp found in the K-7, relying instead on the built-in flash or the AF assist function that's found in some of the company's external flash strobes, such as the AF360FGZ and AF540FGZ. The built-in flash makes a great AF-assist light, as it is very bright, but it requires that the flash head be raised for it to work, so it isn't available for available-light photography. It's also not available in Live View contrast-detect and face-detection AF modes, as the flash duration is too short to be of much assistance during focusing, and it will also not fire in AF.C servo mode (see below).

As noted elsewhere in this review, the K-x lacks any precise indication of AF point location or selection in its viewfinder, with only a set of frame marks that indicate the outer limits of the area covered by the AF system, and an in-focus indictor in the info display area along the bottom of the viewfinder.

Unlike the K-7, the Pentax K-x's focus mode switch only offers two selections: AF (autofocus) and MF (manual focus); however the AF.S (Single) and AF.C (Continuous) servo modes offered on the K-7's AF lever are still available on the Pentax K-x, but are selected via a menu option. AF.S mode locks focus once acquired, and won't refocus unless you remove your finger from the shutter release and half-press it again. The shutter cannot be released unless the camera has acquired focus in this mode. In AF.C mode, autofocus in performed continuously while the shutter release is half-pressed, which is designed to track a moving subject. Unlike AF.S mode, the shutter can be released without acquiring focus when the shutter button is fully depressed in AF.C mode. The Pentax K-x offers a third mode not found on the K-7, called AF.A (Auto), which automatically switches between Single and Continuous mode, depending on subject motion.

The Pentax K-x offers four AF area modes: 5-area Auto, 11-area Auto, Select, and Center. This is up one mode compared to the K-7, which offers just Auto, Select, and Center. In 5-area Auto mode, the camera will select from among the five inner AF areas, arranged in a cross pattern (see animation above). In 11-area mode, it can select from among all 11 AF points. In Select mode, the user selects the specific AF point using the four-way controller and the diagram shown on the LCD. In Center mode, only the center point AF area is used. As mentioned above, there is no indication in the viewfinder of which AF area has been selected or is in focus.

Finally, the Pentax K-x lacks the ability to fine-tune both body and lenses to prevent slight issues with back- or front-focusing.

With the exception of the absence of viewfinder AF indicators, we doubt Pentax's target customers for the K-x will be perturbed by most of these differences. The lack of any way to confirm which AF point the camera has automatically selected, or to manually select AF points without removing your eye from the viewfinder is a disappointing omission. The good news is that the Pentax Kx is a little faster to autofocus than the K2000, and faster in Live View mode than even the K7. It also beats cameras like the Nikon D300S and Canon 7D in its full autofocus lag score, in good light. Pretty impressive.

The Catch-in Focus mode found on the K-7 is also supported by the K-x. This mode, which is accessed via Custom Menu 21, automatically trips the shutter once a subject comes into focus at a pre-determined focal point. AF.A or AF.S servo mode must be selected, and either a manual focus or a DA or FA lens set in MF mode must be used.

Shake Reduction. The K-x includes Pentax's body-based Shake Reduction technology. Unlike lens-based systems, the Pentax K-x's in-body stabilization functions with pretty much any lens that can be attached to the camera. Shake Reduction comes in very handy when shooting with long zooms without a tripod, as they magnify any blurring from camera shake during exposure. Shake Reduction is also useful when shooting under low lighting with a slightly slower shutter speed. According to Pentax, Shake Reduction gives you the flexibility of up to 4 stops slower shutter speed without risking blurring from camera movement; though realistically, exposures longer than 1/15 second typically turn out best with a tripod or other method of camera stabilization. Shake Reduction requires some communication from the lens, particularly the focal length setting. However, for lenses that cannot communicate with the Pentax K-x, you can set the focal length from 8 to 800mm through a setting in the Record menu.

The Shake Reduction function is enabled or disabled through the Control Panel display or page four of the Record menu, and will automatically display the Shake Reduction menu if the camera has no feedback from the lens. Note that Shake Reduction isn't recommended for tripod shooting, and will automatically disable in the self-timer modes, remote control modes, Bulb mode, HDR mode, and when using a wireless flash. It's also not recommended when using very low shutter speeds.

Unlike the system used in the Pentax K-7 (but in common with every other sensor-shift camera on the market), the K-x's shake reduction doesn't correct for rotation, only for horizontal and vertical motion. The drawbacks to sensor shift Shake Reduction technology as used by Pentax and several other companies are twofold. One is that you can't see its effects as you look through the optical viewfinder, as you can with Canon and Nikon's lens-based stabilization systems. But thanks to the Pentax K-x's Live View mode, you can indeed see the effect on the LCD, and SR seems to be pretty solid and effective. The other potential disadvantage is that sensor shift designs aren't necessarily as effective at correcting for shake on lenses with longer focal lengths, compared to lens-based stabilization. At least one third party does offer lens-based stabilization for Pentax digital SLRs, so the potential is there to simply disable the body's own shake reduction and use the lens's stabilization if available. We'll be interested to see if other third parties -- or indeed Pentax themselves -- begin to offer stabilized lenses for Pentax's KAF2 mount, something that could give Pentax users the best of both worlds: lens-based stabilization for long focal lengths, and body-based to allow stabilization with the remainder of the user's lens collection.

Dust Reduction. The K-x also includes a Pentax dust removal system, but it's the previous generation, rather than the updated DR II system that debuted in the Pentax K-7. Like past models, the K-x relies on a combination of SP coating on the low-pass filter to keep dust from adhering to the sensor, and vibration using the sensor shift mechanism to try and remove dust from the sensor. Our past testing has found this to be rather ineffective, compared to systems like that in the K-7 which instead use a piezo-ceramic element to vibrate the low-pass filter. The Pentax K-x does at least offer a Dust Alert function which makes it easy to spot dust on the sensor by taking a test shot with optimal exposure and high contrast for detecting dust visually, making it easier to find and manually remove dust particles. The mirror can be locked up for this purpose in a Sensor Cleaning mode found in page four of the Setup menu.

Despite the dust cleaning features, we haven't seen an automatic system yet that's capable of removing all dust. So while this is a nice feature to have, don't be fooled into thinking that you won't have to either learn how to clean your sensor or send the camera in for cleaning.

If you've got dust specks on your sensor (and sooner or later you will), you're going to need to clean it. There are a lot of products out there intended to address this need, but a distressing number of them work poorly (if at all), and many are grossly overpriced. Advertising hype is rampant, with bogus pseudo-scientific jargon and absurd product claims. And prices -- Did I mention prices? How about $100 for a simple synthetic-bristle brush?

So how do you know what product to use?

We don't pretend to have used everything currently on the market, but we can tell you about one solution that worked very well for us. The "Copper Hill" cleaning method is straightforward and safe, and in our routine usage here at Imaging Resource, highly effective. Better yet, the products sold by Copper Hill Imaging are very reasonably priced. Best of all, Nicholas R (proprietor of Copper Hill) has put together an amazingly detailed tutorial on sensor cleaning, free for all.

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Distortion / Lateral Chromatic Aberration Correction. The Pentax K-x can correct for both lens distortion and lateral chromatic aberration in-camera, when using DA, DA L, and D FA lenses without any accessories between the lens and camera body. This is a feature usually found on more expensive camera models, and Distortion correction corrects both pincushion and barrel distortion for all except the DA Fish-Eye 10-17mm lens, helping ensure lines that should be straight appear so in the final image. Lateral chromatic aberration correction aims to fix color fringing and blurring which can be especially prominent with some lenses toward the corners of the image. While they're only relevant to Raw shooting if you're using Pentax's bundled Digital Camera Utility 4 software or processing the images in-camera, JPEG shooters gain the benefit of both corrections regardless of the software they're using on their computer. Both corrections slow down the camera's burst shooting capabilities significantly, though. This need only be an issue for JPEG shooters. Raw shooters can leave both effects disabled at capture-time to improve the burst speed, and then later apply them in images that require correction when developing the Raw files in-camera. See our test results below to see how well both types of lens correction work.


Pentax K-x Optical Test Results

Below are the results of our optical tests on the Pentax K-x, tested with the 18-55mm f/3.5-5.6 kit lens included in the product bundle.

Kit Lens Test Results

Pretty good performance with the Pentax 18-55mm kit lens.

18mm 55mm

The Pentax K-x is usually sold bundled with a 18-55mm f/3.5-5.6 lens. The kit lens offers a typical optical zoom range of approximately 3x, translating to about 27-83mm when converted to 35mm equivalent. Detail and sharpness were very good across the frame full wide-angle, with low levels of coma distortion and minor softening in the corners, but some fairly strong chromatic aberration is visible in the corners. Results at full telephoto were also good, with very little corner softness and much lower levels of chromatic aberration.

An average sized macro area with the 18-55mm lens, with soft detail. Flash throttled down well up close.

Macro with 18-55mm Lens Macro with Flash

Like zoom range, macro performance will depend entirely on the lens being used. With the Pentax 18-55mm lens, the K-x captured an average size minimum area, measuring 2.53 x 1.68 inches (64 x 43 millimeters). Detail was a quite soft across the frame, and even softer in the extreme corners. (Most lenses have some softening in the corners at macro distances.) The K-x's flash did a good job at throttling down for the macro shot, resulting in an even, albeit slightly dim exposure.

Geometric Distortion
About average distortion at wide-angle and at telephoto with the 18-55mm lens.

Barrel distortion at 18mm is 0.8 percent
Pincushion distortion at 55mm is 0.2 percent

The Pentax 18-55mm lens produced about 0.8 percent barrel distortion at wide-angle, which is about average and noticeable in some of its images. At the telephoto end, there was about 0.2% pincushion distortion, also about average but still noticeable in some shots. Geometric Distortion is the tendency for the lens to bend straight lines outward (like a barrel -- usually at wide-angle) or inward (like a pincushion -- usually at telephoto).

Camera JPEG, Distortion Correction Enabled
Barrel distortion at 18mm is 0.1 percent
Pincushion distortion at 55mm is almost nonexistent

The Pentax K-x does not apply any geometric distortion correction to JPEGs by default, as uncorrected RAW files have the same amounts. There is however a menu option to turn Distortion Correction on. The image above shows wide angle with Distortion Correction enabled. The Pentax K-x reduced the 0.8% barrel distortion into about 0.1% at wide angle, and reduced the pincushion distortion to less than 1 pixel at telephoto.

Chromatic Aberration and Corner Sharpness
Moderately high chromatic aberration at wide-angle, but low at telephoto with the 18-55mm lens. Soft corners on the right-hand side at wide-angle.

Wide: bottom right
C.A.: Moderately high and bright
Softness: Strong blurring
Wide: center
C.A.: Very low
Softness: Slightly soft
Tele: bottom left
C.A.: Low
Softness: Slightly soft
Tele: center
C.A.: Low
Softness: Slightly soft

Chromatic Aberration. Chromatic aberration is moderately high at the full wide-angle setting of the Pentax 18-55mm lens. At telephoto, chromatic aberration is much lower and hardly noticeable. (This distortion is visible as a slight colored fringe around the objects at the edges of the field of view on the resolution target.) The Pentax K-x does not apply any chromatic aberration correction to JPEGs by default, so uncorrected RAW files have similar amounts. There is however a menu option to turn Lateral Chromatic Aberration Correction on (see below).

Corner Softness. The Pentax 18-55mm lens produced soft corners on the right-hand side of the frame at full wide-angle, though softness didn't extend far into the frame. The left side was sharper. At telephoto, corners were a little soft, with the bottom left corner being slightly softer than the rest. The center wasn't as sharp at telephoto as it was at wide-angle. These are typical results for a kit lens, considering the aperture was wide-open for these shots. (Corner sharpness and vignetting typically improve as the lens is stopped-down from maximum aperture.)

Camera JPEG, Lateral Chromatic Aberration Correction
Disabled Enabled

As illustrated by the crops above, the Pentax K-x was reasonably effective at removing the color fringing with Lateral Chromatic Aberration Correction enabled, though it is still detectable.


The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Pentax K-x Photo Gallery .

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