Pincushion effect correction
If the zoom lens of your digicam suffers from barrel distortion at the wide end of the zoom, chances are pincushion distortion will be present at the tele setting.
Just like barrel effect distortion, pincushion effect is most visible on straight lines near the sides of your image frame. Straight lines that pass through the center are unaffected but near the edges they curve outward towards the extreme corners of the image. A rectangle will in fact look like a pincushion.
This is not so-called perspective distortion that results when the camera is tilted, but a true optical aberration that cannot be reduced by using wider or smaller apertures.
In portraits or when taking pictures of natural objects without obvious straight lines the effect is hardly noticeable, but with architectural subjects it can be problematic.
Some camera manufacturers include software to correct this type of distortion or you could use a Photoshop plug-in from Panorama Tools, which uses a high-quality sampling algorithm and allows for effective barrel and pincushion correction
Correction is performed by shifting each pixel radially. By how much is calculated by using a polynomial function based on the specifications of the particular lens used. Once these have been set they can be used for correction of each image taken with that particular lens.