12 mm
Inspired by Ken Rockwell's distortion test for Digital Wide Zooms, I wondered how well PTLens would correct this distortion. Since the Bibble raw converter that I use has a free PTLens plugin, PTLens distortion correction is enabled by default for all my images. Only distortion that can't be corrected by PTLens bothers me. PTLens does a more advanced correction than the Photoshop (two parameters per lens). Apart from comparing the lenses, this also compares the two methods of correcting geometric distortion.
I corrected the images for all lenses that had a usable PTLens profile: Nikon, Tokina and Sigma. The only Tamron profile available for PTLens is for 1,6x crop bodies, which aren't usable with 1,5x crop bodies like the Nikon D200 used for this test.
All images were provided by Ken Rockwell, I used the uncorrected and corrected by the Photoshop Lens Distortion filter from Ken Rockwell, and created the PTLens corrected pictures from Ken Rockwell's original images.
I used PTLens version 7.4.1 with Photoshop CS2 (version 9.0.1). These are the steps that I performed on the PTLens corrected images:
12 mm
15 mm
18 mm
20 mm
24 mm
12 mm (note two humps which are difficult to correct.)
15 mm (no correction needed)
18 mm
20 mm
24 mm
12 mm
15 mm
18 mm
20 mm
24 mm
12 mm
15 mm
18 mm
20 mm
24 mm
12 mm
15 mm
18 mm
20 mm
24 mm (no correction needed)
12 mm
15 mm
18 mm
20 mm
24 mm
10 mm
12 mm (note two humps which are difficult to correct.)
14 mm
18 mm
20 mm
10 mm (at + 2.00, which leaves the cornerscurved.)
10 mm (at +5.00 trying to correct the corners. Note dip in the middle correcting this much.)
12 mm (best without correction.)
14 mm
18 mm
20 mm
10 mm
12 mm
14 mm
18 mm
20 mm
There is some slight distortion visible in the PTLens corrected picture of the Nikon 12-24 at 12mm and 15mm. The Tokina 12-24 has hardly any distortion after correction at any focal length. The Sigma 10-20 has a small amount of visible distortion at 10mm, few distortion (only visible with a ruler) at 12mm, and basically no distortion above 12mm.
The PTLens corrected picture is clearly better than the Photoshop Lens Distortion filter corrected picture for the Sigma 10-20. It's also slightly better with pictures from the Nikon lens. The Tokina pictures were fairly well corrected by the Photoshop filter, so the PTLens is not a visible improvement.
The Tokina has the least amount of distortion after correction with PTLens. The Sigma and Nikon are about equal (small amount of distortion visible at the wide end). I don't think that the distortion in any of the PTLens corrected pictures is a problem in real life; how often do you use a ruler to check if the horizon is straight? Especially if there are less prominent straight lines accross the whole frame. The PTLens correction is clearly superior to the Photoshop Lens Distortion filter. It's at least as good, and often better, than the Photoshop Lens Distortion filter, but easier to use: it reads the Lens and the focal length from the EXIF information, you only have to choose between the Nikon 12-24 and the Tokina 12-24 since they have the same EXIF information, but it remembers this choice. It costs only $10, but works only on Windows. The Bibble plugin works on Windows, Mac and Linux however.
Written by Alson van der Meulen