Pentax 50mm F1.4 comparison: M, FA and DFA
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 three lenses deliver good distortion control. The M 50 shows the highest amount of distortion, at 1.4%. In general this will not be a problem but could benefit from correction in some cases. This value is high for a normal lens. The FA impresses, with 0.6% of distortion, which won't be noticeable in any realistic scenario. The D FA* offers the best results 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 FA offers an excellent 0.6% while the M lens produces an acceptable not average 1.4%.