Pentax-DA 18-55mm F3.5-5.6 WR Review
Distortion is a form of optical aberration in which the magnification of the lens varies across its surface. In photographic lenses, the most common types of distortion are barrel and pincushion distortion. Barrel distortion refers to a decrease in magnification towards the periphery of the lens' surface, while pincushion distortion refers to the opposite effect. In zoom lenses, barrel distortion seems to be typically present in the lower focal length ranges and pincushion distortion, in the upper focal length ranges.
We will be testing the distortion of the 18-55mm in this portion of the review. To achieve that, we photograph a test target with equally-spaced rectangles (in this case, a brick wall) at various focal lengths. We used a K-5 body with its distortion correction feature disabled. The resulting images are as follows:
To have a more objective measurement of the distortion in this lens, we then used the Geometric Distortion feature in the Lens Correction Filter of Adobe Photoshop CS6. In order to fully correct the above images, the following correction values were necessary:
|Geometric Distortion Value||+3.85||+2.95||0||-1||-1.75|
A positive value indicates barrel distortion, a negative value indicates pincushion distortion, and zero means that there is negligible distortion to be corrected. As it can be seen both in the test images and the Geometric Distortion value, the DA 18-55mm WR has very significant barrel distortion at its wide end, reaches a neutral transition point at 35mm, and then gravitates slightly to a pincushion distortion at the long end of the focal length range.
It is typical for inexpensive zoom lenses to have barrel distortion at lower focal lengths and pincushion distortion at its higher focal lengths. In this regard, we were not surprised by the seemingly mediocre distortion performance of the SMC Pentax-DA 18-55mm f/3.5-5.6 AL WR. We can conclude that distortion is clearly a weakness at the wide end.