Thursday, October 26, 2006

Why you should correct for barrel distortion

All zoom lenses are prone to a type of distortion known as barrel distortion, where straight lines appear curved as if the center of the image had been inflated. This is especially the case with the less expensive zoom lenses; usually more expensive lenses, lenses with less zoom range and prime (fixed) lenses are corrected and are closer to a perfcet rectilinear lens. My new lens, the Nikkor 18-200mm, shows a lot of barrel distortion at 18mm.

Fortunately, quite a few tools are available to correct for this predictable type of distortion. I use clens, a command-line utility distributed with panotools, with the lens profiles that come with PTlens, a windows alternative to clens (you can find the database in panotools or here).

The top image above is a straight grid, as shot. You can see the curves that should be straight. The bottom image shows the automatic correction with clens, with the lens profile from PTlens.

clens is run like this:
clens -p profile.txt -s nona -l "Nikkor 18-200mm f/3.5-5.6 G ED-IF AF-S VR DX" image.jpg

If it run like this:
clens -p profile.txt -s nona --Lenses image.jpg

it outputs a list of possible lenses, according to the make of the camera and the focal length.
The correction is not perfect, since the Nikkor 18-200mm shows a more complicated (higher order) type of distortion that clens cannot correct for.


At 12:49 AM, David Naylor said...

I use PTLens. It works fine most of the time, but sometimes it doesn't correctly identify the lens used for taking the photo. That means I can't batch process my photos, but have to sit there checking it gets the lenses right for each photo... :|

It should be possible for it to identify lenses better, since the lens model or zoom range always is specified in the Exif info.


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