Pentax K-S2 Optics

Zoom
A slightly shorter than average kit lens, with above average far-field optical performance.

With an optical zoom ratio of only 2.78x, the Pentax 18-50mm lens offers a little less telephoto reach than typical ~3x zoom kit lenses, though the retracting design makes the lens more compact than the average 18-55m kit lens. 35mm-equivalent zoom range is about 27-75mm.

Sharpness and contrast are pretty good across much of the frame at wide angle (18mm, 27mm eq.), though some corners are soft, even stopped down to f/8. Some chromatic aberration is visible in the corners, even though the K-S2 suppresses it by default in JPEGs. In the wide-angle shot above the building was overexposed by the K-S2's evaluative metering resulting in some very bright highlights, but flare appears to be well-controlled. Performance at 34mm (51mm eq.) and f/8 is quite good, with very good sharpness and contrast across most of the frame, and low C.A. Results at full telephoto (50mm, 75mm eq.) are pretty good with good sharpness and contrast across the frame along with very low C.A, though it's not quite as sharp as the medium focal length. Still, above average far-field performance. See below for our lab results.

Macro
A larger than average macro area with good detail in the center. Flash throttled down well at closest range.

Like zoom range, macro performance will depend entirely on the lens being used. With the Pentax 18-50mm kit lens, the K-S2 captured a larger than average minimum macro area (for an SLR kit lens), measuring 2.92 x 1.95 inches (74 x 49 millimeters). Performance is good in the center of the frame at f/8, with just slightly soft details, though corners are quite soft. (Most lenses have some softening in the corners at macro distances.) The Pentax K-S2's built-in flash is tall enough so that the lens didn't cast a shadow at closest distance, and it throttled down well, producing a slightly dim but fairly evenly exposed image at closest focus distance.

Geometric Distortion
Higher than average distortion at wide angle with the 18-50mm lens.

The Pentax 18-50mm kit lens produces about 0.9 percent barrel distortion at wide angle, which is well above average and noticeable in its images when not corrected. At the telephoto end, there is less than 0.2% pincushion distortion, which is hardly noticeable. 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 18-50mm lens in JPEGs; much higher in uncorrected RAW files. Some very soft corners at wide angle.

Chromatic Aberration. Chromatic aberration is moderate in the corners at full wide angle with the Pentax 18-50mm lens, though like most cameras these days, the K-S2 suppresses lateral chromatic aberration by default in JPEGs. The center shows very low levels. At full telephoto, chromatic aberration is fairly low in the corners, and very low in the center. (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 18-50mm lens produced some soft corners, particularly at wide angle. The left corners are quite soft at wide angle when wide open, and softness extends a fair distance into the frame. The top right corner is fairly sharp, though, while the bottom right is in-between, indicating some decentering. The center was reasonably sharp, though not tack sharp. Corner performance was better at full telephoto, with all four corners only slightly soft. The center was fairly sharp, though again, not tack sharp.

Corner Shading. Moderate amounts of corner shading ("vignetting") can also be seen from the darker corner crops when wide-open, particularly at wide angle.

Stopped-down to f/8. Corner sharpness improved when stopped-down to f/8, but the left corners are still blurry with more obvious chromatic aberration correction reducing edge acuity, producing almost a double image effect. Sharpness in the center is slightly better than wide open but it's still not tack sharp even though sharpening is a little higher. Corners also improved slightly at full telephoto, but again, they are not tack sharp, nor is the center. Corner shading improved at both ends when stopped down to f/8, though some moderate shading is still visible at wide angle.

Built-in Lens Corrections

The Pentax K-S2 offers Distortion Correction, Lateral Chromatic Aberration Suppression, Diffraction Correction 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-S2 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 lens profiles.

The crops above show JPEGs taken with the kit lens at wide angle and telephoto with Distortion Correction enabled. As you can see, the 0.9% barrel distortion in the uncorrected file has been corrected to less than 0.1% complex (slightly moustache-shaped) distortion. At the telephoto end, the 0.2% pincushion distortion in the uncorrected image was corrected to well under 0.1% complex 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 18-50mm exhibits some very high lateral chromatic aberration in the corners when uncorrected (right) at the wide-angle end, and stopping down doesn't help much. At full telephoto, uncorrected C.A. is moderately low, but still more than in corrected images.

Pentax K-S2'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 fringes, and can cause reduced edge acuity as well.

Note that we did not test Diffraction Correction, however it's enabled by default and the camera does appear to apply higher sharpening at smaller apertures to help compensate for diffraction.

Peripheral Illumination Correction

The Pentax K-S2 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 18-50mm kit lens at wide angle and telephoto at maximum aperture. PIC is disabled by default.

Overall, average to slightly above average optical performance for an inexpensive kit lens (especially if you get one that is well centered), and the Pentax K-S2's comprehensive built-in lens corrections help make the most of it.

Coverage
Excellent accuracy from both the optical viewfinder and LCD.

The Pentax K-S2's optical viewfinder shows almost exactly 100% coverage with our Sigma 70mm f/2.8 prime lens. In Live View mode, the LCD provides just over 100% coverage. Excellent performance here.