HD Pentax-D FA* 85mm F1.4 ED SDM AW
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 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 D FA* 85mm. The slider shows the effect of distortion correction on the images.
The amount of distortion is negligible, at 0.2%. It will have absolutely 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.
At 0.2%, distortion is nearly absent with the D FA* 85mm F1.4.