Perspective correction
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Perspective correction or perspective control is a procedure for composing or editing photographs to better conform with the commonly accepted distortions in constructed perspective. The correction would:
- make all lines, which are vertical in reality, vertical on the image. This includes columns, vertical edges of walls, lampposts. Note that this is a commonly accepted distortion in constructed perspectives; perspectives are based on the notion that more distant objects are represented as smaller on the page; however, even though the top of the cathedral tower is in reality further from the viewer than base of the tower (due to the vertical distance), a constructed perspective considers only the horizontal distance and considers the top and bottom to be the same distance away
- make all parallel lines (such as four horizontal edges of a cubic room) cross in one point
Perspective projection distortion occurs in photographs when the film plane is not parallel to lines that are desired to be parallel in the photo. A common case is when a photo is taken of a tall building from ground level by tilting the camera backwards; the building appears to fall away from the camera.
Compare the two images to the right. The first suffers from perspective distortion - in the second that distortion has been corrected.
The popularity of amateur photography has made distorted photos made with cheap cameras so familiar that many people do not immediately realise the distortion. Keep in mind that this "distortion" is relative only to the accepted norm of constructed perspective (where vertical lines in reality do not converge in the constructed image), which in itself is distorted from a true perspective representation (where lines vertical in reality would begin to converge above and below the horizon as they become more distant from the viewer).
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[edit] Perspective correction in the darkroom
A darkroom technician can correct perspective distortion in the printing process. It is usually done by exposing the paper at an angle to the film, with the paper raised toward the part of the image that is larger, therefore not allowing the light from the enlarger to spread as much as the other side of the exposure.
[edit] Perspective correction during digital post-processing
Digital post-processing software provides relatively easy means to correct converging verticals and other distortions introduced at image capture. Adobe Photoshop has several "transform" options to achieve, with care, the desired correction without any significant degradation in the overall image quality. The easiest of these alternatives is the "perspective crop" tool. Photoshop software (CS and subsequent releases) enables the user to perform perspective correction with the cropping tool, setting each side of the crop to independently-determined angles. This is more intuitive and gives the user more control than did earlier perspective-only correction filters.
Other software with mathematical models on how lenses and different types of optical distortions affect the image can correct this by being able to calculate the different characteristics of a lens and re-projecting the image in a number of ways (including non-rectilinear projections). An example of this kind of software is the panorama creation suite Hugin.
[edit] Perspective correction at exposure
Professional cameras where perspective correction is important correct the perspective at exposure by raising the lens parallel to the film. There is more information on this in the view camera article.
Most large format (4x5 and up) cameras have this feature, as well as plane of focus control built into the camera body in the form of flexible bellows and moveable front (lens) and rear (film holder) elements. Thus any focal length lens mounted on a view camera or field camera, and many press cameras can be used with perspective correction.
Some interchangeable lens medium format, 35mm SLR, and Digital SLR camera systems have PC, shift, or tilt/shift lens options which allow perspective correction and, in the case of a tilt/shift lens, plane of focus control, but only at a specific focal length.
[edit] Perspective correction in virtual environments
Architectural images are commonly "rendered" from 3D computer models, for use in promotional material. These have virtual cameras within to create the images, which normally have modifiers capable of correcting (or distorting) the perspective to the artist's taste.
