The apparent convergence of vertical lines at the top of a picture frame is most noticeable when a camera is tilted up to include the top of a tall building. This is caused because the back of the camera, and the film plane, are no longer parallel with the subject. The base of the structure is closer to the film and seems larger; the top is farther away and appears smaller. This results in the sides of the building appearing to converge at the top. By moving back from the subject, you may be able to get all the building in without tilting the camera, but details might then look small. Using a shift lens is the solution to this problem.
A shift, or perspective control (PC), lens alters the appearance of vertical lines. By turning a knob on the lens you can make the front of the lens shift upward, downward, or sideways. So instead of tilting the camera to include the top of a building, simply shift the lens upward until the top of the building comes into view on the focusing screen. Shift lenses are costly, are only available for certain 35mm and digital SLRs, and usually have a fixed wide-angle focal length.