Sigma 17-70mm F2.8-4 Contemporary
Distortion
It is extremely difficult to design an optical lens that projects images perfectly onto a flat image plane such as the sensor in a DSLR. Therefore, most lenses exhibit some type of distortion: either barrel distortion (where lines that are straight in real life bend outward on the image) or pincushion distortion (where lines that are straight in real life bend inward). Zoom lenses tend to exhibit one type of distortion at one end of the range and the other at the other lens; furthermore, zooms also tend to have more distortion than primes. While the Sigma 17-70mm is no exception to this rule, it still does a good job of controlling distortions.
To evaluate the 17-70mm's distortion, we took a photo of a brick wall at key focal lengths, and the applied the necessary distortion correction using Adobe Photoshop to objectively measure the distortion.
17mm
24mm
28mm
35mm
50mm
70mm
Verdict
The Photoshop adjustments needed to correct each image are summarized in the table below:
Focal Length | Correction |
| 17mm | +3.6 |
| 24mm | -0.8 |
| 28mm | -1.2 |
| 35mm | -2.2 |
| 50mm | -1.7 |
| 70mm | -2.0 |
The 17-70mm exhibits moderate barrel distortion at 17mm which quickly turns in to pincushion distortion around 20mm. The lens is at its best between 20mm and 28mm, though pincushion distortion continues to be noticeable through 70mm.
Overall, this particular lens exhibits approximately the same amount of distortion as both of its predecessors. It still outperforms super-zoom lenses such as the Pentax 18-135mm or 18-270mm, however. Because it isn't a genuine Pentax lens, however, it does not support in-camera distortion correction, which may be an issue for those seeking to print out-of-camera images.
In practice, the 17-70mm's distortion was only a minor issue. Distortion correction was not applied to most of our sample photos at the end of this review.
