Shift lenses are high-quality lenses, usually wide-angle, that provide a parallel shift facility like the sliding lens panel of professional large-format cameras for correcting converging vertical lines and manipulating the perspective especially for use in architectural and product photography.
Whereas normal lenses designed for 35mm full-frame cameras have an image circle diameter of 43.27mm so that all four corners of the image are inside the image circle, shift lenses provide much larger image circle (60mm or even more). Decentration of the lens is possible within this area.
Vertical shift is the most popular: upward when photographing high buildings, and downward for product shots, so that the camera does not have to be tilted. When the camera is tilted either upward or downward, perpendicular lines are not imaged as perpendicular, but rather converge upward or downward, which is very pronounced in wide-angle shots and can be very irritating.