HD Pentax-D FA 70-210mm F4 ED SDM WR

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 files with a Pentax lens.

Distortion Test Results

The following images show how the distortion pattern looks when photographed with the D FA 70-210mm, for APS-C and full frame sensors. The slider shows the effect of distortion correction on the images.

70mm Full Frame

135mm Full Frame

210mm Full Frame

The amount of distortion is surprising, even with in-camera corrections activated. At all focal lengths, the lens exhibits pincushion distortion. The following table summarizes the results.

Verdict

Distortion is sadly quite prominent with the D FA 70-210mm. It is not out of control but occurs at all focal lengths and even though it can be corrected after the fact, it has an effect on images straight out of the camera.