Pentax K100D Optics

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Pentax K100D Optics

The Pentax K100D features a Pentax K AF 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 K100D'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 K100D's autofocus system uses a TTL phase-matching system (called SAFOX VIII by Pentax) with 11 available AF points spread throughout the central portion of the frame. A switch on the left side of the lens barrel (as viewed from the rear) puts the camera into auto or manual focus modes. Through the LCD menu system, you can opt for Single or Continuous AF modes as well, with the Continuous option adjusting focus constantly for moving subjects. There's also a menu option 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.

Because the Pentax K100D 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 two steps 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 K100D, 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.

K100D: 18-55mm Kit Lens Optical Tests

Zoom
Good performance from the Pentax K100D's 18-55mm kit lens.


18mm	55mm

The Pentax K100D is available bundled with the Pentax SMC-DA 18-55mm f/3.5-5.6 kit lens. The K100D works with a huge array of K-mount lenses. Results are good at wide angle, with a slight softness throughout the frame. Some coma distortion and chromatic aberration in the corners, as expected. The lens shows a good deal more detail at telephoto, and is fairly contrasty. This far field performance differs slightly from what you'll see below in the Corner Softness section, where the wide angle lens is sharper in the center and corners and softer at telephoto (which is precisely why we shoot a "far" shot, because lenses are often quite different focused near compared to far).

Macro
A small macro area with good detail. Flash performs well up close.


Standard Macro	Macro with Flash

As with zoom performance, the Pentax K100D's macro performance will depend on the lens in use. However, with the 18-55mm kit lens set to 55mm, the K100D captured a small minimum area measuring 2.58 x 1.72 inches (66 x 44 millimeters). Details are a little fuzzy throughout the frame, with some additional softness in the corners from the lens, but resolution is still good. There's a slight reddish color cast from the automatic white balance setting. The flash exposure looked pretty good, with nearly even coverage (though some slight falloff in the lower corners).

Distortion
Moderate barrel distortion, and slightly high pincushion.

This 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). The Pentax K100D's 18-55mm kit lens produced about 0.72% barrel distortion at wide angle, which is actually slightly less than average among the cameras we've tested, although still noticeable in its images. At the telephoto end, the lens produced about 0.15% pincushion distortion, which is slightly more than average.

Barrel distortion at 18mm is 0.72%

Pincushion at 55mm is less than 0.15%

Chromatic aberration
Moderate to very low distortion with the 18-55mm kit lens.


Wide: moderate but bright, top left @ 200%	Wide: bright, top right @ 200%

Tele: low, top left @200%	Tele: no visible pixels, top right @200%

Chromatic aberration with the Pentax K100D's kit lens is moderate at the 18mm setting, though fairly bright, showing about 5-6 pixels. At 55mm, the distortion is very slight in the top left corner, and virtually invisible in the top right. (This distortion is visible as a very slight colored fringe around the objects at the edges of the field of view on the resolution target.)

Corner Sharpness
Slight softening in the corners with the 18-55mm kit lens.


Wide: Slightly soft in the lower corners (lower left)	Wide: sharper at center, though still slightly soft

Tele: slightly soft in the corners (lower left)	Tele: still slightly soft at center

The Pentax K100D's 18-55mm kit lens produced slightly soft corners in a few shots. At wide angle, corners were a hint soft, particularly in the lower corners, compared to the center of the frame. At telephoto, corners showed stronger blurring, but the center was also a bit soft. Still, not a bad performance.

Viewfinder

Coverage
Good coverage from the optical viewfinder, but offset and tilted somewhat.


18mm, optical viewfinder	55mm, optical viewfinder

The Pentax K100D's optical viewfinder proved reasonably accurate, showing about 98% coverage at wide angle and about 97% coverage at telephoto, which is better than average for a consumer model. The resulting images were however vertically offset as well as slightly tilted, which unfortunately is quite common in inexpensive SLRs.

The images above were taken from our standardized test shots. For a collection of more pictorial photos, see our Pentax K100D Photo Gallery .