Radial Distortion: Correcting Pincushion
Radial Distortion is one of the most common “diseases” of inexpensive and common camera lenses, especially the zoom lenses. Typically, there are two types of distortion: pincushion and barrel distortion. The pincushion distortion is the most common one. In this case, the lines that do not pass from the image center tend to curve inward. If you use a grid to take pictures, you'll understand this feature, since this type of distortion gives the so called pillow effect.
The Pincuchion Distortion
Pincushion distortion occurs when you use a zoom lens or when you are trying to take a picture using a telephoto or super zoom lens extended to the longest possible focal length. Although this problem can occur even when using a simple fixed lens, the most drastic example of aberration occurs when using a zoom lens that has more elements and groups for the image and light to go through.
Correcting the Pincuchion Distortion
Correcting Pincushion distortion is not easy, but is not impossible either. There are many sophisticated applications available today in the market, able to deal with such issues. They make use of correction algorithms with simple measuring and correcting tools, such as Photoshop Elements, Lightroom or CS4. Using these programs, you can manually fix the problem, adjusting the distortion. There are also numerous free options with a distortion fixing ability, and others that offer automatic correction options such as DxO Lab Software.
If you are taking landscape pictures or long distance pictures and you cannot avoid using a zoom lens at full focal length, you should always set the lens measurement points correctly to avoid irreversible distortion. However, even high distortion can be tolerated and corrected when using the right piece of software and application.