Some cameras (normally but not exclusively large-format models) have tilt lenses that are attached to the camera body using bellows. They can be angled up or down, or from side to side, relative to the plane of focus and hence the normal plane of the film or sensor. This facility is most commonly used to increase the depth of field in images, for instance when photographing prepared food. Some cameras also have a tilting back which allows the film plane to be moved relative to the lens axis to achieve a broadly similar effect.
Lenses are normally attached to cameras with their optical axes perpendicular to the plane of the film or sensor. When the lens is focused at a particular distance everything in the plane parallel to the film or sensor at the focused distance is then rendered sharp. Depth of field is determined by the focusing distance, the aperture and the focal length of the lens.
When a lens axis is angled away from the normal perpendicular alignment, the plane of focus is no longer parallel to the recording medium. With care, this characteristic can be used to achieve significantly increased depth of field because different parts of the image are brought in to sharp focus at various distances from the rear element of the lens. As the lens tilts so the plane of focus also tilts. However, the relationship between the two rotations is complex and a tilt of just a few degrees for the lens creates a much larger tilt in the plane of focus. The so-called Scheimpflug rule states that the film plane, the lens plane and the plane of sharp focus intersect at a common point. Complicated mathematical calculations, impractical in the field, are required to predict the precise effects.
Exposure determination also becomes more complicated when a lens is tilted because automatic systems are designed to work with the lens axis perpendicular to the plane of the film or sensor. Once this relationship is changed, the photographer must intervene manually. Bracketing may be required.