This technique sometimes is referred to as "anti-Scheimpflug", though it actually relies on the Scheimpflug principle. Rotation of the plane of focus can be accomplished by rotating either the lens plane or the image plane. Rotating the lens (as by adjusting the front standard on a view camera) does not alter linear perspective in a planar subject such as the face of a building, but requires a lens with a large image circle to avoid vignetting. Rotating the image plane (as by adjusting the back or rear standard on a view camera) alters perspective (e.g., the sides of a building converge), but works with a lens that has a smaller image circle. Rotation of the lens or back about a horizontal axis is commonly called tilt, and rotation about a vertical axis is commonly called swing. Camera movements Tilt and swing are movements available on most view cameras, often on both the front and rear standards, and on some small- and medium format cameras using special lenses that partially emulate view-camera movements. Such lenses are often called tilt-shift or "perspective control" lenses. For some camera models there are adapters that enable movements with some of the manufacturer's regular lenses, and a crude approximation may be achieved with such attachments as the 'Lensbaby' or by 'freelensing'. Depth of field When the lens and image planes are parallel, the depth of field (DoF) extends between parallel planes on either side of the plane of focus. When the Scheimpflug principle is employed, the DoF becomes wedge shaped (Merklinger 1996, 32; Tillmanns 1997, 71), with the apex of the wedge at the PoF rotation axis, as shown in Figure 5. 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. The shallow DoF near the camera requires the PoF to be positioned carefully if near objects are to be rendered sharply. On a plane parallel to the image plane, the DoF is equally distributed above and below the PoF; in Figure 5, the distances yn and yf on the plane VP are equal. This distribution can be helpful in determining the best position for the PoF; if a scene includes a distant tall feature, the best fit of the DoF to the scene often results from having the PoF pass through the vertical midpoint of that feature. The angular