HD Pentax-D FA 150-450mm F4.5-5.6
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 150-450mm has a relatively short zoom range (3X) but long focal lengths in absolute terms. As such, it is expected that it should display some pincushion distortion at most if not all focal lengths.
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 correction for our tests. 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:
150-450mm | |
150 mm | -1.5% |
300 mm | -0.9% |
450 mm | -0.5% |
There is some distortion in our uncorrected images at all positions. However, it should be mostly unnoticeable for focal lengths longer than 300mm. At 150mm, the distortion will be visible in the right circumstances.
In-camera distortion correction completely removes the distortion in all cases.
The following images show the how our distortion test patterns look like uncorrected.
150mm
300mm
450mm
Verdict
Distortion is generally well controlled, with the worst results occurring at 150mm. It will mostly be unnoticeable for focal lengths longer than 300mm. In all cases, distortion is completely removed by either in-camera correction or via software.