Tilt-shift Photography

A camera lens can provide sharp focus on only a single plane. Without tilt, the image plane (containing the film or image sensor), the lens plane, and the plane of focus are parallel, and are perpendicular to the lens axis; objects in sharp focus are all at the same distance from the camera. When the lens plane is tilted relative to the image plane, the plane of focus (PoF) is at an angle to the image plane, and objects at different distances from the camera can all be sharply focused if they lie in the same plane. With the lens tilted, the image plane, lens plane, and PoF intersect at a common line; this behavior has become known as the Scheimpflug principle. When focus is adjusted with a tilted lens, the PoF rotates about an axis at the intersection of the lens's front focal plane and a plane through the center of the lens parallel to the image plane; the tilt determines the distance from the axis of rotation to the center of the lens, and the focus determines the angle of the PoF with the image plane. In combination, the tilt and focus determine the position of the PoF.
When the PoF coincides with an essentially flat subject, the entire subject is sharp; in applications such as landscape photography, getting everything sharp is often the objective.
The PoF can also be oriented so that only a small part of it passes through the subject, producing a very shallow region of sharpness, and the effect is quite different from that obtained simply by using a large aperture with a regular camera.
Using tilt changes the shape of the depth of field (DoF). When the lens and image planes are parallel, the DoF extends between parallel planes on either side of the PoF. With tilt or swing, the DoF is wedge shaped, with the apex of the wedge near the camera, as shown in Figure 5 in the Scheimpflug principle article. The DoF is zero at the apex, remains shallow at the edge of the lens's field of view, and increases with distance from the camera. For a given position of the PoF, the angle between the planes that define the near and far limits of DoF (i.e., the angular DoF) increases with lens f-number; for a given f-number and angle of the PoF, the angular DoF decreases with increasing tilt. When it is desired to have an entire scene sharp, as in landscape photography, the best results are often achieved with a relatively small amount of tilt. When the objective is selective focus, a large amount of tilt can be used to give a very small angular DoF; however, the tilt fixes the position of the PoF rotation axis, so if tilt is used to control the DoF, it may not be possible to also have the PoF pass through all desired points.