Digital Photographer's Software Guide - Bertemes - Net

23
Lens Distortion Correction
Lens and perspective distortion are among the most common “errors” in photography.
In one sense they are not really errors at all because they are almost impossible to avoid
completely. Tilt the camera up and you get parallel lines converging toward the top of
the image, tilt it down and you get keystone distortion. If you fit a wide-angle lens, especially
on a full frame DSLR, you will undoubtedly see some barrel distortion at the
edges. Lens manufacturers make corrections to barreling and its opposite pincushion
effect using aspherical elements, but sometimes this results in a double-whammy, in socalled
“complex distortion,” which can create a wavy image not unlike the shape of Sir
Arthur Conan Doyle’s moustache.
The most extreme distortion comes from the use of ultra-wide-angle or fisheye lenses. For
these 14mm and smaller focal length lenses, manufacturers have dropped any pretense of
eradicating the distortion and so it becomes an integral part of the image. At least, in precomputer
days, that is how it was treated—as a dramatic effect to show (for example) the
supposedly gigantic pair of feet of someone lying down, head away from the camera. But
now that the computer can unwrap the image to give it a true perspective, the use of ultrawide
and fisheye lenses has changed completely. You can photograph an interior with a
fisheye and make it suitable for the human eye in, almost, the blink of an eye.
For very rapid results, the software ideally needs access to specific data about the
lens/camera combination used for taking the image. Armed with this information, it
can easily make corrections by running the right set of numbers. Without it, you have
to start using slider controls while judging the results in real-time.
Correcting geometric errors can make an improvement to the image equal to that of
exposure correction or white balance. It is so important it should not be left to the standard
tools of an image editor although good facilities are available in Photoshop CS2
to CS4 and Paint Shop Pro. Specialist packages are listed in this chapter. You will also