Perspective control lens
In photography, a perspective correction lens (also known as a shift lens or PC lens, or a tilt and shift lens or TS lens if the lens offers the tilt feature) is used to correct the optical effect of perspective. This type of lens is mostly used in architectural and other technical photography. Perspective control lenses are generally made for single-lens reflex (SLR) 35mm and medium-format SLR cameras, as most large format cameras allow for perspective correction using movements.
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| Tilting the camera upwards results in vertical perspective |
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| Keeping the camera level captures only the bottom portion of the building |
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| Shifting the lens upwards results in a good picture |
When making photographs of buildings or other large structures from the ground, it is usually impossible (without PC) to fit the entire building in the frame without tilting the camera upwards. The resultant perspective causes the top of the building to appear smaller than its base, which is often considered undesirable. The perspective effect is proportional to the lens's angle of view.
Photographing from the ground, perspective can be eliminated by keeping the film plane parallel to the building. However, with most lenses, this results in capturing only the bottom part of the subject, as the rest is cropped out. With a perspective control lens, however, the lens may be shifted upwards in relation to the film plane, placing the complete subject in the frame. The ground level, the camera's point of perspective, is shifted to the bottom of the frame.
Another use of shifting is in taking pictures of a mirror. By shifting the camera off to one side of the mirror, and shifting the lens in the opposite direction, an image of the mirror can be captured without the reflection of the camera or photographer. Shifting can similarly be used to photograph 'around' objects, such as a building support in a gallery, without changing perspective.
In the 35mm film format, PC lenses for architectural photography have appeared in focal lengths of 24mm, 28mm, and 35mm. The earliest such lenses were those of 35mm, but those proved too long for photographing buildings in urban environments. Accordingly, lenses of 28mm and then 24mm became available and were quickly adopted by photographers because of their ability to photograph buildings successfully in relatively cramped urban settings.
A perspective control 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 film plane cannot be shifted outside of the image area. However, all PC lenses require a small aperture setting to prevent vignetting when significant shifts are employed. PC lenses for 35mm cameras typically offer a maximum shift of 11mm.
PC lenses are all prime lenses. They can be quite expensive compared to regular prime lenses. Some medium format camera makers, such as Mamiya, have addressed this problem by offering shift adapters that work with the maker's other prime lenses.
In the production of computer images of architectural projects, an equivalent correction is employed in the same manner within the software.
References
- "What is perspective correction?". f-stop.com. Retrieved 2006-05-22.
- Sheeran, Frank (1997). "How Shift Lenses Change your Life". photo.net. Retrieved 2006-05-22.