Optical Distortion explained

Optical distortion is a serious problem, low-cost or high quality lenses produce distortion. This post explains the origins of these anomalies, how to reduce their results while shooting or using computer programmes.

Aberrations are caused by the shape and the design of a lens. These aberrations lead to optical distortion which does not affect so much some subjects, for example, landscapes. Optical distortion, instead, makes obvious in buildings or in highly geometric structures such as chess boards. The degree of distortion a lens produces does not depend on lens focal length: so we have the same grid pattern from 18mm up to 300mm. However, wide-angle lenses usually originate stronger optical distortion than tele-lenses.

Example of barrel distortion effects

There are few different kinds of distortion: the most famous is barrel effect. Every lens has a centre point of anomalies, often in the centre of the frame. The straight lines, which do not pass through the center, are not parallel and the image is captured curved inward toward the middle of the frame by the lens and the effect is an optical distortion called barrel. On the other hand, the opposite result is pincushion that makes straight lines seems to curve outward from the image centre. The most common is barrel distortion; this problem frequently affects wide lens (Fisheye) or high-range lenses: low-cost optics with the focal lenght from 18mm to 200mm or higher create an evident barrel distortion while pincushion is prevalent in telephotography lenses.

Distortion correction techniques

As there are no distortion-free lenses, the produced anomalies should be reduced manually or automatically after shooting. Unfortunately, processing an image for distortion correction results in cropping the image: this makes the final picture smaller than the initial and it may cause data loss.

Example of pincushion distortion results

Many high level cameras, also DSLR ones, have built-in distortion correction mechanisms. One of the first cameras, that has introduced such techniques, is Nikon D90 and now all vendors add them to their cameras.

Computer-based distortion correction has provided by some tools such as PanoTools since 1998. Now picture editing programmes, such as Adobe Photoshop, have error correction functionality to remove optical distortion.

  • Using Corel Pain Shop Pro Photo, you open Setting menu and select Barrel distortion correction or fisheye distortion correction or pincushion one.
  • On Photoshop CS2 or CS3 or CS4, your choose Filter menu, then Distort, finally Lens Correction. While using Photoshop CS5, distortion reduction tools are in Filter menu and then Lens Correction.

 

In the following example a distortion correction measure is applied by Corel software. As can be seen, the picture on the right is less sharp than the original on the left, but the barrel effect is less marked.

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