Shape Control
When the camera back is parallel to a planar subject (such as the front of a building), all points in the subject are at the same distance from the camera, and are recorded at the same magnification. The shape of the subject is recorded without distortion. When the image plane is not parallel to the subject, as when pointing the camera up at a tall building, parts of the subject are at varying distances from the camera; the more distant parts are recorded at lesser magnification, causing the convergence of parallel lines. Because the subject is at an angle to the camera, it is also foreshortened.
When the camera back is not parallel to a planar subject, it is not possible to have the entire subject in focus without the use of tilt or swing; consequently, the image must rely on the depth of field to have the entire subject rendered acceptably sharp.
With a PC lens, the camera back can be kept parallel to the subject while the lens is moved to achieve the desired positioning of the subject in the image area. All points in the subject remain at the same distance from the camera, and the subject shape is preserved. If desired, the camera back can be rotated away from parallel to the subject, to allow some convergence of parallel lines or even to increase the convergence. Again, the position of the subject in the image area is adjusted by moving the lens.
Read more about this topic: Perspective Control Lens
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