perspective control (shift) lens

When photographing architectural subjects it is usually necessary to keep the camera film plane vertical in order to avoid keystoning (converging verticals) effects. To achieve this without cutting off part of the subject matter, field and studio cameras have a facility for shifting the lens panel so that the centres of the subject, the lens, and the film format are in a straight line. Since the 1980s, so-called perspective control (or ‘shift’) lenses have become available for 35 mm and medium-format cameras, the amount of shift being controlled by a setscrew. As the mechanism operates in only one direction, the entire lens mount is rotatable in steps. More recently, lenses that can also be rotated about their rear nodal point have become available. These permit the application of the Scheimpflug rule to the photography of scenes in which the important planes of the subject and the image are not parallel.

— Roger W. Hicks