HD Pentax-DA* 16-50mm F2.8 ED PLM AW

Distortions

Distortion refers to the capability of a lens to represent straight lines as...straight lines. Wide angle lenses often create barrel distortion, where lines look they've been put on a sphere. Telephoto lenses can create the reverse effect, named pincushion distortion. The following images illustrate the various types of distortion:

Barrel distortion, left; no distortion, center; pincushion distortion, right

It is very difficult to create a zoom without some distortion. It is also very hard to design a wide lens, even a prime, which is distortion-free. On the other hand, normal and short tele lenses can more easily avoid the problem.

The way to evaluate distortion is pretty straightforward: take a picture of straight regular lines, and look to see if they curve. Calculating the ratio of curvature yields an evaluation of distortion.

Cameras such as the K-3 and K-1 include built-in distortion correction, which can be automatically applied to files captured with a Pentax lens.

Distortion Test Results

The following images show how the distortion pattern looks when photographed with the 16-50mm, for various focal lengths. The slider shows the effect of distortion correction on the images.

16mm

30mm

50mm

The following table summarizes numerically the results.

At 16mm, distortion is prominent at 2.5%. It will be noticeable with straight lines near the edges. At medium focal lengths, it becomes low enough to be negligible. At the tele end, it is more important, but will probably not be bothersome in most cases.

These results are not surprising for a zoom lens, not worrisome but nothing particularly impressive either.

Modern cameras include an automatic distortion correction tool, and most post-processing programs will take care of it without trouble.

Verdict

Distortion will be present and visible at 16mm, but negligible at medium focal lengths. At 50mm it is present but will rarely be noticeable.