When the lens axis is perpendicular to the image plane, as is normally the case, the Plane of Focus (POF) is parallel to the image plane, and the DOF extends between parallel planes on either side of the POF.
When the lens axis is not perpendicular to the image plane, the POF is no longer parallel to the image plane; the ability to rotate the POF is known as the Scheimpflug principle.
Rotation of the POF is accomplished with camera movements (tilt, a rotation of the lens about a horizontal axis, or swing, a rotation about a vertical axis).
Tilt and swing are available on most view cameras, and are also available with specific lenses on some small- and medium-format cameras.
When the POF is rotated, the near and far limits of DOF are no longer parallel; the DOF becomes wedge-shaped, with the apex of the wedge nearest the camera.
With tilt, the height of the DOF increases with distance from the camera; with swing, the width of the DOF increases with distance.
Rotating the POF with tilt or swing (or both) can be used either to maximize or minimize the part of an image that is within the DOF.
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