Pentax K-30 Digital Camera Review
The 18-135mm f/3.5-5.6 lens that we tested the K-30 with struggled to corral chromatic aberration through much of the focal and aperture range away from the center of the image. When the aperture was as small as f/40 at 135mm the diffraction made the image look blurred near the edged while the center remained sharp. In addition, lateral chromatic aberration resulted in purple fringing when shooting wide open or close down, both at the telephoto focal length of 135mm and the wide angle 18mm focal length.
Distortion was a pretty constant issue throughout testing with the 18-135mm WR lens. As with most zoom lenses that extend from wide to telephoto lengths, distortion ranged from extreme barrel warping at the wide end to a pincushion distortion in the middle and telephoto lengths. According to our testing the 18mm focal length resulted in 2.73% of barrel distortion, while the lens quickly switched that to more than 1.5% of pincushion distortion through the rest of the focal range. These are correctable flaws, but it's worth noting if you plan to photograph interiors or buildings for architecture photography.