This photo can be used to illustrate several types of “distortions” usually associated with ultra-wide angle lens (UWA).
Barrel distortion makes straight lines look like curves, especially near the edges. Consumer quality zoom lenses with large zoom ratios usually have this problem at the wide end. As you can see here, the 10-22 has barely discernable barrel distortion. Although no distortion correction was performed, all the straight lines look straight enough. For critical architectural works, PTLens is a wonderful tool for correcting distortion, including 3rd degree complex “mustache” lines.
The second type of distortion is the converging parallel lines. It is caused by the sensor of the camera not parallel to the plane of your subject. It is most disturbing when shooting upward causing converging vertical lines that make buildings look leaned back, as shown in the photo on the right. In the above main photo, the camera is aimed slightly downward. The vertical lines, although straight, are not parallel, but will converge below the photo. Because my camera is not parallel to any of the walls (or the ceiling), the horizontal lines on the walls and the ceiling would not appear parallel. Post processing software such as PhotoShop or PTLens include perspective correction tools to straighten out parallel lines, such as the example on the right.
The third type of distortion is the perspective of being close. The size of foreground object is exaggerated. When you try to fill the frame with a face, you get a comical big nose. The oblong shape on my son’s head above is NOT caused by being close, but is the next type of distortion.
The forth type of distortion is sometimes called Volume Anamorphosis. It makes 3D objects near the edges stretched out towards the corners. Ironically, lens with low barrel distortion makes this distortion worst. What makes a brick wall look flat will flatten a spherical object into an oval. Both the front and side of my son’s head are not that much out of proportion. But he looks really fat when they are flattened out. This effect happens regardless of distance. It gives my wife large feet. Although I stood further away, it made me look fat also :) The persons in the photo near right are distorted by both closeness and near the edges. The people in the large group far right are distorted by being near the edge only. However, by adding barrel distortion, the heads can be made to look more natural as in this photo. Click below to see more examples of the difference before and after adding barrel distortion.

Elevator to the top of the Observation Tower
deYoung Museum