Pincushion distortion is a lens effect which causes images to become pinched in the center. It is most often associated with telephoto lenses, and in particular, zoom telephotos. The distortion will usually occur at the telephoto end of the lens. The pincushion distortion effect increases with the distance the object is from the optical axis of the lens.
It is the opposite effect to barrel lens distortion and, like its counterpart, pincushion distortion is most visible in images with straight lines (especially when the lines are close to the edge of the image).
The look of pincushion distortion is similar to a looking at piece of paper with grid lines on it, and with the center of the paper pushed inward. If you're shooting a photo of a building with straight lines, for example, the edges of the building will appear curve inward when pincushion distortion occurs.
Fixing Pincushion Distortion
Pincushion distortion can be corrected easily in modern editing programs such as Photoshop, which contains a "lens distortion" correction filter. Free editing programs also offer slightly less sophisticated corrections.
Like Barrel distortion, pincushion distortion is amplified by the effects of perspective on images.
Because pincushion distortion most commonly affects objects with straight lines, you can minimize the effect of pincushion distortion by shooting objects that don't have straight lines. Additionally, try to avoid shooting at the maximum magnification of the telephoto zoom lens. If you have straight lines in your object, try to keep them as close to the center of the frame as possible.