Modern 50mm Shootout: Pentax D FA* 50mm, Samyang 50mm, Pentax DA* 55mm F1.4
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 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 three tested lenses. The slider shows the effect of distortion correction on the images.
D FA* 50mm
The DA* 55mm presents a reasonable amount of distortion, at 1.1%. For the most case, it won't be visible in the field. The effect is stronger on the Rokinon, at 1.8%. It will be noticeable in some cases, and since this is a third-party lens, in-camera automatic corrections are not possible. The D FA* again takes the lead, with an almost-absent 0.15% of distortion, which is probably the lowest value we've ever measured.
The D FA* presents near-perfect distortion figures, at 0.15%. The DA* keeps the effect reasonably under control, at 1.1%, while the Rokinon lags, with 1.8% of distortion.