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Perspective control is a procedure for composing or editing photographs to better conform with the commonly accepted distortions in constructed perspective. The control would: * make all lines that are vertical in reality vertical in the image. This includes
columns A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression member ...
, vertical edges of walls and lampposts. This is a commonly accepted distortion in constructed perspective; perspective is 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), 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 distortion occurs in photographs when the film plane is not parallel to lines that are required 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. __TOC__


At exposure

Professional cameras where perspective control is important control the perspective at exposure by raising the lens parallel to the film. There is more information on this in the
view camera A view camera is a large format, large-format camera in which the large format lens, lens forms an erect image, inverted image on a ground glass, ground-glass screen directly at the film plane. The image is viewed, composed, and focused, then the ...
article. Most
large format Large format photography refers to any imaging format of or larger. Large format is larger than "medium format", the or size of Hasselblad, Mamiya, Rollei, Kowa, and Pentax cameras (using 120 film, 120- and 220-roll film), and much la ...
(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 A field camera is a view camera that can be folded in a compact size. Modern designs are little different from the first folding field cameras from the 19th century. In general they have more limited View camera#Movements, camera movements than ...
, and many
press camera A press camera is a medium or large format view camera that was predominantly used by press photographers in the early to mid-20th century. It was largely replaced for press photography by 35mm film cameras in the 1960s, and subsequently, by ...
s can be used with perspective control. Some interchangeable lens
medium format Medium format has traditionally referred to a film format in photography and the related cameras and equipment that use film. Nowadays, the term applies to film and digital cameras that record images on media larger than the used in 35&n ...
, 35 mm film SLR, and Digital SLR camera systems have PC, shift, or tilt/shift lens options which allow perspective control and, in the case of a tilt/shift lens, plane of focus control, but only at a specific focal length.


In the darkroom

A
darkroom A darkroom is used to process photographic film, make Photographic printing, prints and carry out other associated tasks. It is a room that can be made completely dark to allow the processing of light-sensitive photographic materials, including ...
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. The process is known as rectification printing, and is done using a rectifying printer (transforming printer), which involves rotating the negative and/or easel. Restoring parallelism to verticals (for instance) is easily done by tilting one plane, but if the focal length of the enlarger is not suitably chosen, the resulting image will have vertical distortion (compression or stretching). For correct perspective correction, the proper focal length (specifically, angle of view) must be chosen so that the enlargement replicates the perspective of the camera.


During digital post-processing

Digital post-processing software provides means to correct converging verticals and other distortions introduced at image capture.
Adobe Photoshop Adobe Photoshop is a raster graphics editor developed and published by Adobe Inc., Adobe for Microsoft Windows, Windows and macOS. It was created in 1987 by Thomas Knoll, Thomas and John Knoll. It is the most used tool for professional digital ...
and
GIMP Gimp or GIMP may refer to: Clothing * Bondage suit, also called a gimp suit, a type of suit used in BDSM * Bondage mask, also called a gimp mask, often worn in conjunction with a gimp suit Embroidery and crafts * Gimp (thread), an ornamental tr ...
have several "transform" options to achieve, with care, the desired control without any significant degradation in the overall image quality. Photoshop CS2 and subsequent releases includes perspective correction as part of its Lens Distortion Correction Filter;Correcting Perspective in Photoshop
Ken Rockwell
DxO Optics Pro from DxO Labs includes perspective correction; while GIMP (as of 2.6) does not include a specialized tool for correcting perspective, though a plug-in
EZ Perspective
is available.
RawTherapee RawTherapee is a free and open source application for processing photographs in raw image formats such as those created by many digital cameras. It comprises a subset of image editing operations specifically aimed at non-destructive post-produc ...
, a free and open-source raw converter, includes horizontal and vertical perspective correction tools too. Note that because the mathematics of projective transforms depends on the
angle of view In photography, angle of view (AOV) describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term '' field of view''. It is important to distinguish the angle of view from the ...
, perspective tools require that the angle of view or
35 mm equivalent focal length In photography, the 35 mm equivalent focal length is a measure of the angle of view for a particular combination of a camera lens and film or image sensor size. The term is popular because in the early years of digital photography, most p ...
be entered, though this can often be determined from
Exif Exchangeable image file format (officially Exif, according to JEIDA/JEITA/CIPA specifications) is a standard that specifies formats for images, sound, and ancillary tags used by digital cameras (including smartphones), scanners and other system ...
metadata.Hugin tutorial — Perspective correction
/ref> It is to correct perspective using a general
projective transformation In projective geometry, a homography is an isomorphism of projective spaces, induced by an isomorphism of the vector spaces from which the projective spaces derive. It is a bijection that maps lines to lines, and thus a collineation. In general, ...
tool, correcting vertical tilt (converging verticals) by stretching out the top;Perspective Distortion Correction Tutorial
Lone Star Digital

Larry N. Bolch
Perspective Adjustment in PhotoShop
The Luminous Landscape
this is the "Distort Transform" in Photoshop, and the "perspective tool" in GIMP. However, this introduces vertical distortion – objects appear squat (vertically compressed, horizontally extended) – unless the vertical dimension is also stretched. This effect is minor for small angles, and can be corrected by hand, manually stretching the vertical dimension until the proportions look right, but is automatically done by specialized perspective transform tools. An alternative interface, found in Photoshop (CS and subsequent releases) is the "perspective crop", which enables the user to perform perspective control with the cropping tool, setting each side of the crop to independently determined angles, which can be more intuitive and direct. 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. However these techniques do not enable the recovery of lost
spatial resolution In physics and geosciences, the term spatial resolution refers to distance between independent measurements, or the physical dimension that represents a pixel of the image. While in some instruments, like cameras and telescopes, spatial resoluti ...
in the more distant areas of the subject, or the recovery of lost
depth of field The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus. Factors affecting depth ...
due to the angle of the film/sensor plane to the subject. These transforms involve
interpolation In the mathematics, mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points. In engineering and science, one ...
, as in
image scaling In computer graphics and digital imaging, image scaling refers to the resizing of a digital image. In video technology, the magnification of digital material is known as upscaling or resolution enhancement. When scaling a vector graphic image ...
, which degrades the image quality, in particular blurring high-frequency detail. How significant this is depends on the original image resolution, degree of manipulation, print/display size, and viewing distance, and perspective correction must be traded off against preserving high-frequency detail.


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. See
3D projection A 3D projection (or graphical projection) is a Design, design technique used to display a three-dimensional (3D) object on a two-dimensional (2D) surface. These projections rely on perspective (graphical), visual perspective and aspect analysi ...
for details.


See also

*
Anamorphosis Anamorphosis is a distorted projection that requires the viewer to occupy a specific vantage point, use special devices, or both to view a recognizable image. It is used in painting, photography, sculpture and installation, toys, and film speci ...
*
Keystone effect The keystone effect is the apparent distortion of an image caused by projecting it onto an angled surface. It is the distortion of the image dimensions, such as making a square look like a trapezoid, the shape of an architectural keystone, hence ...
*
Image distortion In geometric optics, distortion is a deviation from rectilinear projection; a projection in which straight lines in a scene remain straight in an image. It is a form of optical aberration that may be distinguished from other aberrations such a ...


References


External links


Illustrations

Panorama Tools wiki page on perspective control


* ttp://hugin.sourceforge.net/tutorials/perspective/ Correcting perspective using the Open Source Hugin software {{DEFAULTSORT:Perspective Control Photographic techniques Perspective projection Composition in visual art