Pentax K-7 Optics
Lens Compatibility. The Pentax K-7 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 K7's broad range of lenses. (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, and FA zoom lenses with power zoom are fully supported.
The table below (courtesy of Pentax USA), details the K-7's lens compatibility with the various types of Pentax lenses.
Pentax K-7 Autofocus System
Compared to the K20D, the new Pentax K-7 feels faster and more responsive in virtually every respect. One area of particular improvement is its AF system. The AF system in the K20D was called the SAFOX VIII; the new system is called the SAFOX VIII+. The main AF sensor is the same 11-point unit used in the K20D, but the AF algorithms have been considerably enhanced, and a light-color sensor has been added that improves focus accuracy under light sources with widely varying hues.
Our strong subjective impression has been that the Pentax K-7 achieves focus much more quickly in difficult situations of low light and/or low subject contrast than does the K20D. This contributed greatly to the sense of responsiveness we felt when using the Pentax K-7.
Illuminant-Color Autofocus Correction. We found the idea of illuminant-color focus adjustments particularly intriguing, but Pentax didn't have a lot of details for us on just what was being done. From the limited information we received, though, we think we've arrived at a pretty good understanding of what's involved.
It's a fundamental law of optics that light is refracted varying amounts depending on its color (wavelength). Anyone who's put a prism in the path of a beam of light and observed the resulting rainbow has experienced this firsthand. In camera optics, great pains are taken to force all the different colors of light to end up bending by the same amount, so they can come to focus at the same point. This accounts for much of the complexity of modern apochromatic lenses. In a camera's phase-detection AF system, though, there isn't room (or budget) for fancy apochromatic optics; simple prisms have to suffice to split the incoming light and divert it to the AF sensor pixels. This means that incoming reddish light won't be bent as much as bluish-tinted light would be. AF systems are adjusted to produce accurate results with daylight-balanced lighting, where all the colors are more or less equally represented. This works well in most situations, but when faced with a strongly-tinted light source, an AF system can misjudge the point of correct focus. Two commonly-encountered extremes are the very warm hue of household incandescent lighting, or the cool bluish tones of open shade outdoors. By measuring the overall tint of the ambient lighting, the AF system in the Pentax K7 can compensate for these errors, and deliver more accurate focusing under a wide range of light colors.
Dedicated AF-Assist Lamp. Previously, the K20D and other Pentax SLRs used the flash head as the AF-assist lamp in situations where the ambient light levels were too low for the AF system to operate reliably. The flash made a great AF-assist light, as it was very bright, but it required that the flash head be raised for it to work, so it wasn't available for available-light photography. The Pentax K-7 now has a dedicated AF-assist light, a small, very bright green LED located just to the right of and below the shutter button, as you look at the front of the camera. Because of its close proximity to the lens barrel, some larger lenses and those with wide lens hoods may block the light from reaching the subject, but you can generally dispense with lens hoods for available-light photography, and when we did so, we found that the AF-assist light worked fine with lenses ranging from the 15mm f/4 Limited model to the 17-70mm f/4 DA SDM. (The latter being one that required removal of its lens hood, though.) Long tele zooms might not work with it, but then the AF-assist lamp's light won't project far enough for telephoto subjects anyway.
AF Adjustment. If we've discovered anything reviewing lenses on SLRgear.com, it's that lenses and bodies don't always match. Sometimes they focus in front of the subject, sometimes they focus in back of the subject. Camera companies are starting to acknowledge this, building in adjustments to compensate for front- and back-focusing problems. The Pentax K7 has the same system as the K20D, does just that, and it doesn't just work for the body side of the equation. Sometimes it's the lenses that are out of tune too, so adjusting just the camera's AF to work well with one lens won't solve the problem with another; indeed it can make other lenses worse. The Pentax K7 allows the body to be adjusted (a default or "global" setting), and up to 20 lens types can be registered with their own AF adjustment. Adjustment range is +/-10 arbitrary steps.
Shake Reduction. Because the Pentax K7 hosts a wide range of lenses, including those with long zooms, the camera offers Pentax's body-based Shake Reduction technology. 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 approximately 2.5 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-7, you can set the focal length from 8 to 800mm through a setting in the Record menu. The Shake Reduction switch on the rear panel enables the function, 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 mode, Bulb mode, and when using a wireless flash. It's also not recommended when panning.
For the first time, the sensor-shift mechanism can correct not only for horizontal and vertical motion, but also for rotation around the axis of the lens barrel (though we've reported that this was a feature of Shake Reduction back to the K100D, Pentax informs us that this was a result of a translation error back in 2006). One degree of rotational correction on either side of the central position is possible.
The drawback to Pentax's Shake Reduction technology 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-7's Live View mode, you can indeed see the effect on the LCD, and SR seems to be pretty solid and effective.
Dust Reduction. The Pentax K-7 also includes a new DR II dust removal system. Where past models relied on the sensor shift mechanism to remove dust from the sensor -- rather ineffectively according to our tests -- the K-7 now includes a piezo-ceramic element to vibrate the low-pass filter. There is also 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. In addition, a special coating is applied to the sensor cover glass to reduce the chances of dust adhering to it.
Pentax K-7 Optical Test Results
Below are the results of our optical tests on the Pentax K-7. While not officially a kit lens (the K-7 is sold body-only at this point), we used the SMC Pentax-DA 17-70mm f/4 AL IF SDM lens for this section, as we think it'll be a popular combination.
Very good performance with the Pentax 17-70mm lens.
The Pentax K-7 is usually sold body-only, but we decided to test it with the 17-70mm f/4 lens for this Optics page of the review. (Other sections of the review use our reference Sigma 70mm f/2.8 prime.) The Pentax 17-70mm lens offers a better than typical optical zoom range of ~4.1x, and is slightly wider than most kit lenses at full wide-angle, at 17mm (about 26mm equivalent). Detail and sharpness are strong across the frame full wide-angle, with low levels of coma distortion but some strong chromatic aberration in the corners. Results at full telephoto are also very good, with very little corner softness and much lower levels of chromatic aberration.
A reasonably large macro area with the 17-70mm lens, with soft detail. Flash has trouble up close.
|Macro with 17-70mm
|Macro with Flash|
Like zoom range, macro performance will depend entirely on the lens being used. With the Pentax 17-70mm lens, the K-7 captured a larger macro area than average, measuring 2.83 x 1.88 inches (72 x 48 millimeters). Detail was a little soft across the frame, and even softer in the extreme corners. (Most lenses have some softening in the corners at macro distances.) The K-7's flash didn't throttle down well for the macro shot, overexposing part of the frame, and the lens barrel casts a large shadow, resulting in a very uneven exposure.
About average distortion at wide-angle, slightly higher than average at telephoto with the 17-70mm lens.
|Barrel distortion at 17mm is 0.8 percent|
|Pincushion distortion at 70mm is 0.3 percent|
The Pentax 17-70mm 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 0.3% pincushion distortion which is slightly higher than average, and 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|
The Pentax K-7 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-7 overcorrected a bit, turning 0.8 percent barrel distortion into about 0.1% pincushion at wide angle.
Moderately high at wide-angle, but low at telephoto with the 17-70mm lens.
|Wide: Moderately high and bright,
top left @ 100 percent
|Wide: Moderately high and bright,
top right @ 100 percent
|Tele: Low and dull,
top left @100 percent
|Tele: Low and dull,
top right @100 percent
Chromatic aberration is moderately high at the full wide-angle setting of the Pentax 17-70mm 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-7 does not apply any chromatic aberration correction to JPEGs by default, as uncorrected RAW files have similar amounts. There is however a menu option to turn Lateral Chromatic Aberration Correction on.
|Camera JPEG, Lateral Chromatic Aberration Correction Enabled|
|Wide: Very low,
top left @ 100 percent
|Wide: Very low,
top right @ 100 percent
As illustrated above, the Pentax K-7 was quite effective at removing the color fringing with Lateral Chromatic Aberration Correction enabled, though if you click on the crops above, you can see it wasn't quite as effective on diagonal edges.
Some softness in the corners of the frame with the 17-70mm lens.
|Wide: Moderate softness in the
corners (upper left).
|Wide: Pretty sharp at center.|
|Tele: Moderate softness in the
corners (lower right).
|Tele: Slightly soft at center.|
The Pentax 17-70mm lens produced slightly soft corners of the frame at full wide-angle. The softness was fairly symmetrical, and didn't extend far into the frame. At telephoto, corners were also soft, with the bottom right corner being slightly softer than the rest. The center wasn't as sharp at telephoto as it was at wide-angle. Still, pretty good results, especially considering the aperture was wide-open at f/4.0. (Corner sharpness typically improves as the lens is stopped-down from maximum aperture.)