HD vs. SMC Pentax-DA 18-50mm F4-5.6

Distortions

Geometric distortion refers to the capability of a lens to represent straight lines in the scene as straight lines in the image. Wide lenses/zoom settings often create what is called barrel distorsion, where lines look spherized. Tele lenses can create the reverse, named pincushion distortion. The following image illustrates the various types of distortion:

Barrel distorsion, left; no distorsion, center; pinsuchion distorsion,right
Barrel distorsion, left; no distorsion, center; pinsuchion 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, that is distorsion-free. On the other hand, normal and short tele lenses can more easily avoid the problem.

On this page, we will be evaluating the distortion performance of the 18-50mm zoom and comparing it to the DA 18-55mm.  For all tests we include the original image as well as one corrected via the in-camera distortion correction option.

Distorsion Test

18-50mm
18-55mm
18mm
Image A
Image B
Image A
Image B
24mm
Image A
Image B
Image A
Image B
35mm
Image A
Image B
Image A
Image B
45mm
Image A
Image B
Image A
Image B

50mm
/
55mm

Image A
Image B
Image A
Image B

The two lenses show similar distortion features at all focal lengths. About 3% barrel distortion is visible at 18mm, which will show in many shooting situations. There is almost no distortion in the middle of the range, and pincushion appears around 35mm, peaking near 1% at the longest focal lengths. That level of distortion will be hard to spot in typical scenes. 

Real-world example

Here is an example showing that in most cases, distortion at the widest angle will not be a problem.

18-50mm at 18mm
18-55mm  at 18mm
18-50mm 18-55mm

Verdict

As expected, the 18-50mm exhibits barrel distortion at the wider end of the zoom range and pincushion distortion beyond 35mm.  However, the geometric distortion is quite low overall and only a practical concern at 18mm, where it can still easily be corrected in-camera or via post-processing software profiles. If left uncorrected, it will be visible in scenes with straight lines near the edges. At most other focal lengths, uncorrected distortion will only have a small or negligible effect in practice.

While the 18-50mm is far from the distortion-free nature of a prime lens, we feel that it fares exceptionally well in this area for a kit lens, given its entry-level price. The 18-55mm performs similarly to its newer sibling.


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