Barrel effect correction
Many cameras with zoom lenses suffer from barrel distortion. The effect causes images to be spherised at their center and it occurs at the wide end of a zoom lens. This type of distortion is brought about by the way modern zoom lenses are constructed and using wider or smaller apertures has no effect on it whatsoever.
It is most noticeable when you have straight edges or lines near the side of your image frame. Straight lines that pass through the center of your image are unaffected, but near the edges they curve away from the extreme corners – a rectangle will look like a barrel. In portraiture or when taking photographs of natural objects that lack obvious straight lines, the effect is hardly noticeable.
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. The amount of which is calculated by using a polynomial function based on the specifications of the particular lens used. Once these have been determined they can be used for correction of each image taken with that particular lens.