Lens pincushion and barrel distortion can be a problem when capturing straight lines close to the edges of a photo with a wide angel lense or a zoom lens in wide angel position. I didn't care too much about correcting such distortions so far, but there is this one photo whose appearance has kept bugging me for months now:
RAG BERGWERK LIPPE/WESTERHOLT [OLD VERSION]
Shaft 1 (left) and shaft 2 (right) of 3 shafts; in service; Gelsenkirchen, Germany; April 2004
Click to enlarge
The banana-shaped train and the banana-shaped winding tower are hard to miss. If we have a look at the original version without perspective correction, the reasons for why things look banana become clear:
ORIGINAL VERSION
Click to enlarge
With a wood starting right behind the photographer's position, the pit had to be captured from a far too close position with the Canon Powershot S40's tiny lens set to its shortest focal length. Taken from a more distant position with dominating horizontal or vertical lines farther away from the edges and the focal lenght set to no more than 95% of its minimum, lens distortions would be much less obvious.
So, how to get rid of the bulge? Try the free PTLens Photoshop plugin (Windows only). "PTLens will examine the JPEG file, determine lens focal length and image orientation from EXIF information, use the focal length to lookup lens correction coefficients in a database, and call [the Panorama Tools DLL] to correct the image." And it works:
RAG BERGWERK LIPPE/WESTERHOLT [NEW VERSION]
Click to enlarge
PTLens comes with a whole range of profiles for digital cameras with built-in lenses and for Nikkor, Canon and Sigma interchangeable lenses. If your model is not on the list, you can send in a sample shots and the author Thomas Niemann will determine the calibration coefficients from it. It crashed Photoshop on first run after installation and then worked flawlessly on all Canon S40, Canon G1 and Olympus C8080 examples tested so far. Just 50 kB large and highly recommended.
Thomas Niemann's PTLens >
