Difference between revisions of "Perspective control lens"

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Revision as of 05:24, 9 February 2012

Shift lenses are produced for SLR cameras to allow perspective correction which is especially useful for architectural photography. Compared to a technical camera, which allows full movement of lens and film back, a shift lens offers only limited horizontal and vertical movement. They are also known as PC (Perspective Correction) or PA (German). Shift lenses are always wide angle and can be used to compensate the converging verticals which result when a lens is pointed upwards.

Construction

In order to allow movement, the lens must be constructed so that image coverage will be larger than the usual image size. This means that the projected image extends beyond the normal frame, usually resulting in a more expensive lens with smaller maximum aperture. The lens itself can be physically shifted (slid by hand or screw adjuster) either vertically, horizontally, or a combination.

In use the camera will be set up on a tripod and pointed at the subject making sure that the line of sight is near horizontal. Then the lens will be shifted upwards bringing the subject down into view without affecting perspective. The effect is exactly the same as moving the point of view (i.e. camera) upwards.

Because of the movement, shift lenses do not have automatic diaphrams and must be used in stop down mode. Also it is usual for the manufacturer to stipulate that smaller apertures should be used when the lens is shifted.

Photo editing software can correct perspective, along with other optical distortion, replacing the need for shift lenses.



Glossary Terms