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In photography, a perspective control lens allows the photographer to control the appearance of perspective in the image; the lens can be moved parallel to the film or sensor, providing the equivalent of corresponding view camera movements. This movement of the lens allows adjusting the position of the subject in the image area without moving the camera back; it is often used to avoid convergence of parallel lines, such as when photographing a tall building. A lens that provides only shift is called a shift lens, while those that can also tilt are called tilt-shift lenses. The terms ''PC'' and ''TS'' are also used by some manufacturers to refer to this type of lens. Short-focus perspective-control (PC) lenses (i.e., 17 mm through 35 mm) are used mostly in architectural photography; longer focal lengths may also be used in other applications such as landscape, product, and closeup photography. PC lenses are generally designed for single-lens reflex (SLR) cameras, as rangefinder cameras do not allow the photographer to directly view the effect of the lens, and view cameras allow for perspective control using camera movements. A PC lens has a larger image circle than is required to cover the image area (film or sensor size). Typically, the image circle is large enough, and the mechanics of the lens sufficiently limited, that the image area cannot be shifted outside of the image circle. However, many PC lenses require a small aperture setting to prevent vignetting when significant shifts are employed. PC lenses for 35 mm cameras typically offer a maximum shift of 11 mm; some newer models offer a maximum shift of 12 mm. The mathematics involved in tilt lenses are described as the Scheimpflug principle, after an Austrian military officer who developed the technique for correcting distortion in aerial photographs. The first PC lens manufactured for an SLR camera in any format was Nikon’s 1961 3.5 35 mm PC-Nikkor; it was followed by an 2.8 35 mm PC-Nikkor (1968), an 4 28 mm PC-Nikkor (1975), and an 3.5 28 mm PC-Nikkor (1981).〔(【引用サイトリンク】 publisher = Nikon Corporation )〕 In 1973, Canon introduced a lens, the TS 35 mm 2.8 SSC,〔 (Canon TS 35 mm 2.8 SSC lens ). (Canon Camera Museum ) 〕 with tilt as well as shift functions. Other manufacturers, including Olympus, Pentax, Schneider Kreuznach (produced as well for Leica), and Minolta, made their own versions of PC lenses. Olympus produced 35 mm and 24 mm shift lenses. Canon currently offers 17 mm, 24 mm, 45 mm, and 90 mm tilt/shift lenses. Nikon currently offers 24 mm, 45 mm, and 85 mm PC lenses with tilt and shift capability. == 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.〔Ray 2000, 171.〕 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.〔 Ray 2000, 172. 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Perspective control lens」の詳細全文を読む スポンサード リンク
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