HD Pentax-D FA 24-70mm F2.8 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.
With a moderate zoom range but a wide field of view at 24mm, out test subject should perform best at the center of its range, and worse at its widest angle.
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. We disabled the distortion correction. The images below illustrate distortion results using a K-1.
Distortion Test results
The following table summarizes the distortion values at three focal lengths for both tested lenses:
24-70mm | |
24 mm | 2.2% |
50 mm | 0.7% |
70 mm | -0.1% |
As expected, 24mm results show the worst performance, which at this level will be visible in some shooting scenarios. This value is high, even for a wide lens. 50mm and 70mm distortion figures are below 1% and should be hard to notice in the field. Note that distortion correction, be it in-camera or via software, will essentially remove any visible distortion, especially with the longer focal lengths.
The following images show the how our distortion test patterns look like uncorrected.
24mm
50mm
70mm
The image below illustrates how distortion can become visible in real life scenarios (the picture is taken at 24mm).
Verdict
The lens shows good control over distortion at 50mm and 70mm, with results that will not be visible in normal use. However, barrel distortion at 24mm is high, at 2.2%, which can be noticeable if the camera's distortion correction is deactivated and no post-processing is applied.