Correcting lens and perspective distortion
When you photograph buildings, or any scene with strong vertical lines, your image will show, to varying degrees, distortions due to the lens and arising from the angle of view. When you print the photograph these distortions are immediately obvious and can ruin an otherwise good print. They can also make building a panorama from several distorted photographs very difficult, even if the distortions are slight.
There are specialist cameras and lenses you can use to avoid these problems, but unless you're a professional photographer, these are probably outside your budget. Fortunately with digital image processing, we can correct most or all of these distortions. There are quite a number of ways to do this, and for different image editing software applications there are different approaches.
This article shows how to do this using Photoshop CS2, PaintShop Pro 8, Fireworks (4, MX, MX2004, 8) and earlier versions of Photoshop and PSP using plugins from theimagingfactory.com.
Overview
The origin of the problem and some definitions
Lens distortions
Lens designers face major problems trying to image straight lines in the real world onto film or image sensor as straight lines without curvature or other distortions. For normal cameras they're also limited by manufacturing costs. So most consumer or 'pro-sumer' grade lenses are a compromise and may display two sorts of distortion - barrel and pincushion. Figure 1 shows how these show up.
Figure 1. Barrel and Pincushion distortion
These distortions are quite often apparent when using a zoom lens, at either extremity of its range (wide angle or telephoto).
David lives in Canberra, Australia. He trained in Upper Atmospheric Physics, but spent longer than he cares to admit as a Science bureaucrat in the Australian Government. He has been building websites since 1997, professionally since 1999. He is the co-author with Linda Rathgeber of "Playing with Fire", contributed the accessibility chapter to Dreamweaver MX Magic, and other bits and pieces. He has academic publications in areas as diverse as astrophysics and fractal ferns. His interests include photography, restoring golf antiques, collecting old
78 records, fern ecology, and he's also a HiFi freak. He teaches astronomy at a local community college."
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