HD Pentax-DA* 11-18mm F2.8 ED DC 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, rightBarrel 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 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 files with a Pentax lens.

Distortion Test results

The following images show how the distortion pattern looks when photographed with the DA* 11-18mm with an APS-C sensor. The slider shows the effect of distortion correction on the images.

11mm

No correction
Corrected

14mm

No correction
Corrected

18mm

No correction
Corrected

When left uncorrected, distortion is quite strong. At 11mm, it reaches 4.9%, a value which is sure to be noticed in most situations. At 14mm it falls to 3.5%, and lastly to 2.1% at 18mm. This latter value is no worse than many other lenses. These results appear on the high side, and indeed they are. However, when compared with other recent ultra-wide lenses from the likes of Irix and Venus Optics, the 11-18mm performs adequately except at 11mm. Moreover, none of these are zooms, and it is to be expected than a zoom will not produce ideal distortion figures at all focal lengths. That the 11-18mm can produce about 2% at 18mm is in itself an achievement. In other words, while distortion is strong at 11mm and significant at 14mm, these results are not too far from expectations.

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

Verdict

Distortion is strong at 11mm, still present at 14mm and mostly controlled at 18mm. It is recommended to use automatic distortion correction tools when using the DA* 11-18mm.


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