SMC Pentax-FA 43mm F1.9 Limited Review

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 like 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 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.

Distortion Test results

The following images show how the distortion pattern looks when photographed with the FA 43mm, for APS-C and full frame sensors. The slider shows the effect of distortion correction on the images.

APS-C

Full frame

The amount of distortion is negligible, at 0.65% both with APS-C and full frame sensors. It should have no effect in actual shooting conditions. Modern cameras include an automatic distortion correction tool, and most post-processing programs will take care of it without trouble.

Verdict

At 0.65%, distortion is negligible with the FA 43mm Limited.