Perspective control

Perspective control is a procedure for composing or editing photographs to better conform with the commonly accepted distortions in constructed perspective. The control would:

Perspective projection distortion occurs in photographs when the film plane is not parallel to lines that are required to be parallel in the photo. A common case is when a photo is taken of a tall building from ground level by tilting the camera backwards: the building appears to fall away from the camera.

In the two images shown to the right, the first suffers from perspective distortion — in the second that distortion has been corrected.

The popularity of amateur photography has made distorted photos made with cheap cameras so familiar that many people do not immediately realise the distortion. This "distortion" is relative only to the accepted norm of constructed perspective (where vertical lines in reality do not converge in the constructed image), which in itself is distorted from a true perspective representation (where lines that are vertical in reality would begin to converge above and below the horizon as they become more distant from the viewer).

Professional cameras where perspective control is important control the perspective at exposure by raising the lens parallel to the film. There is more information on this in the view camera article.

Most large format (4x5 and up) cameras have this feature, as well as plane of focus control built into the camera body in the form of flexible bellows and moveable front (lens) and rear (film holder) elements. Thus any focal length lens mounted on a view camera or field camera, and many press cameras can be used with perspective control.

Some interchangeable lens medium format, 35 mm film SLR, and Digital SLR camera systems have PC, shift, or tilt/shift lens options which allow perspective control and, in the case of a tilt/shift lens, plane of focus control, but only at a specific focal length.

A darkroom technician can correct perspective distortion in the printing process. It is usually done by exposing the paper at an angle to the film, with the paper raised toward the part of the image that is larger, therefore not allowing the light from the enlarger to spread as much as the other side of the exposure.

...
Wikipedia