Pentax K-3 Optics

Zoom
A longer than average kit lens, with average performance.

With a generous optical zoom ratio of 7.5x, the Pentax 18-135mm lens offers much greater telephoto reach than typical ~3x zoom kit lenses. 35mm-equivalent zoom range is about 27-203mm.

Sharpness and contrast are very good in the center of the frame at wide angle, though corners are soft, even stopped down to f/8. Chromatic aberration is quite low, though the K-3 suppresses it by default in JPEGs (see below). Performance at 31mm and f/8 is actually pretty good, with very good sharpness over much of the frame and just a touch of softness in extreme corners. Results at full telephoto are good near the center of the frame at f/8, but corners are quite soft extending deep into the frame.

Macro
A larger than average macro area with somewhat soft detail. Flash throttled down well at closest range.

Like zoom range, macro performance will depend entirely on the lens being used. With the Pentax 18-135mm lens, the K-3 captured a larger than average minimum macro area (for an SLR kit lens), measuring 3.48 x 2.31 inches (88 x 59 millimeters). Details are a bit soft in center of the frame and corners are softer, and there's also some slight shading ("vignetting") in the extreme corners. (Most lenses have some softening and shading in the corners at macro distances.) The Pentax K-3's built-in flash is tall enough so that the lens didn't cast a shadow, and it throttled down well, producing a slightly dim but evenly exposed image at closest focus distance.

Geometric Distortion
Higher than average distortion at wide angle and telephoto with the 18-135mm lens.

The Pentax 18-135mm lens produces about 1.0 percent barrel distortion at wide angle, which is well above average and noticeable in its images. At the telephoto end, there is about 0.5% pincushion distortion, 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).

Chromatic Aberration and Corner Sharpness
Low to moderate levels of chromatic aberration from the 18-135mm lens in JPEGs; much higher in uncorrected RAW files. Very soft corners at both wide angle and telephoto.

Chromatic Aberration. Chromatic aberration is low in the corners at the full wide angle setting of the Pentax 18-135mm lens, because the K-3 suppresses lateral chromatic aberration by default. At full telephoto, chromatic aberration is moderate in the corners, and more 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.) See below for uncorrected results.

Corner Sharpness. The Pentax 18-135mm lens produces soft corners at full wide angle and full telephoto when shot wide-open at maximum aperture. All four corners are soft at wide angle, with the upper left softer than the others. Blurring extends a small to moderate distance into the frame depending on the corner, but the center is reasonably sharp. At full telephoto, corners are also quite soft, with the lower right corner the softest this time. Softness extends quite far into the frame, though the center is fairly sharp.

Corner Shading. Moderate amounts of corner shading ("vignetting") can also be seen from the darker corner crops, at both maximum wide angle and telephoto settings.

Stopped-down to f/8. Corner sharpness improves when stopped-down to f/8, but they are still quite soft. Chromatic aberration remains about the same, perhaps slightly lower and not quite as exacerbated by the blurring in the corners. Corner shading improves quite a bit, though, and center sharpness improves as well.

Overall, about average performance for a kit lens, though keep in mind the 18-135mm does offer much more reach than most bundled lenses.

Built-in Lens Corrections

The Pentax K-3 offers Distortion Correction, Lateral Chromatic Aberration Suppression and Peripheral Illumination Correction lens correction options. Corrections are supported for most DA, DA L, and DFA lenses, as well as several of the company's FA Limited lenses, in both JPEG and RAW workflows. For JPEG shooting, the correction is made at capture time, while in RAW mode, correction information is saved as a file parameter.

Distortion Correction

The Pentax K-3 does not apply any geometric distortion correction to JPEGs by default. There is however a menu option to turn Distortion Correction on. RAW files are not corrected, but are tagged to have the same correction applied when using a RAW converter that supports the embedded parameters.

The crops above show JPEGs taken with the kit lens at wide angle and telephoto with Distortion Correction enabled. As you can see, the 1.0% barrel distortion in the uncorrected file has been slightly overcorrected to about 0.1% complex (slightly moustache-shaped) distortion. At the telephoto end, the 0.5% pincushion distortion in the uncorrected image was overcorrected to about 0.2% barrel distortion. Also notice some of the image has been cropped away in the process, especially at wide angle, so keep that in mind.

Chromatic Aberration Reduction

As illustrated by the crops above, the 18-135mm exhibits some very high lateral chromatic aberration in the corners when uncorrected (right) at the telephoto end, exacerbated by corner softness. At wide angle, C.A. isn't quite as bad, but still much more than in corrected images.

Pentax K-3's processor is reasonably effective at removing much of color fringing with Lateral Chromatic Aberration Correction enabled, though it still leaves some visible chromatic aberration behind in the form of magenta and blue fringes.

Peripheral Illumination Correction

The Pentax K-3 also provides "Peripheral Illumination Correction," which corrects for lens shading (commonly called "vignetting"), attempting to produce a more uniform exposure across the frame by compensating for the light fall-off seen with some lenses in the corners of the frame. Mouse over the Off and On links above to see the effect on the 18-135mm kit lens at wide angle and telephoto at maximum aperture. PIC is disabled by default.

Coverage
Excellent accuracy from both the optical viewfinder and LCD.

The Pentax K-3's optical viewfinder shows essentially 100% coverage vertically, and just under 100% coverage horizontally with our Sigma 70mm f/2.8 prime lens. In Live View mode, the LCD exhibits just over 100% coverage vertically, and close to 100% horizontally. Excellent performance here.