Pentax K20D Optics
Lens Compatibility. The Pentax K20D 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 K20D'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.
Autofocus. The K20D's autofocus system uses the same TTL phase-matching system used on the K10D, called SAFOX VIII by Pentax, which has 11 AF points spread throughout the central portion of the frame. The central 9 are wide cross area sensors, while the outer two to the left and right are vertical line-type sensors. A three-position switch to the left side of the lens mount (as viewed from the rear) puts the camera into Single AF, Continuous AF or Manual focus modes. There's also a switch on the back panel for controlling the main AF point, which offers Auto, Select, and Center options. In Auto mode, the camera automatically determines the main AF point based on the proximity of the subject, while Select lets you choose one of the 11 available points in the spread. Center simply sets the AF point to the very center of the frame. In either case, the selected AF point displays in red in the optical viewfinder, unless this option is disabled through the Custom menu. When using manual focus, you can program the OK button on the rear panel to quickly adjust autofocus, thereby giving you a starting point to manually tweak the focus. The K20D also has a neat Catch-in focus mode, where the camera will wait until a subject arrives at a preset focus point before it captures an image. The Pentax K20D's Live View mode uses the same SAFOX VIII phase-detection autofocus system; no contrast-detection mode (which works off the image sensor's data) is offered. The K20D does not provide a dedicated AF assist lamp, but can use a series of strobes from the built-in flash instead.
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 K20D has a new system that 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 K20D 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 K20D 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 K20D, 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.
Dust Reduction. The Shake Reduction mechanism is also used to vibrate dust from the K20D's CMOS imager. You can activate dust removal manually, or have the camera automatically perform it at power-up. There is also a Dust Alert function which make 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.
The Pentax K20D is usually sold body only, so this section left blank intentionally, as optical performance will depend on the lens being used. To see how the optional Pentax 18-55mm f/3.5-5.6 lens performs, see the Pentax K200D Review's Optics page.