Perspective distortion (photography)
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In photography and cinematography, perspective distortion is a warping or transformation of an object and its surrounding area that differs significantly from what the object would look like with a normal focal length. Perspective distortion can typically be seen in images shot using a wide angle of view, where an object close to the lens appears abnormally large relative to more distant objects, or in distant shots with a narrow angle of view, where the viewer cannot discern relative distances between distant objects and more distant objects may look exceptionally large, when such images are viewed with a typical viewing angle.
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[edit] Influencing factors
Perspective distortion is influenced by the relationship between two factors: the angle of view at which the image is captured by the camera and the angle of view at which the photograph of the subject is presented or viewed.
[edit] Angle of view of the capture
When photographs are viewed at the ordinary viewing distance[1], the angle of view at which the image is captured accounts completely for the appearance of perspective distortion. The general assumption that "undoctored" photos cannot distort a scene is incorrect. Perspective distortion is particularly noticeable in portraits taken with wide-angle lenses at short camera-to-subject distances. They generally give an unpleasant impression, making the nose appear too large with respect to the rest of the face, and distorting the facial expression. Framing the same subject identically while using a moderate telephoto or long focus lens (with a narrow angle of view) flattens the image to a more flattering perspective. It is for this reason that, for a 35 mm camera, lenses with focal lengths from about 85 through 135 mm are generally considered to be good portrait lenses. Conversely, using lenses with much longer focal lengths for portraits results in more extreme flattening of facial features, which also may be objectionable to the viewer.
[edit] Photograph viewing distance
Photographs are ordinarily viewed at a distance approximately equal to their diagonal. When viewed at this distance, the distortion effects created by the angle of view of the capture are apparent. However, theoretically, if one views pictures exhibiting extension (wide angle) distortion at a closer distance, thus widening the angle of view of the presentation, then the phenomenon abates. Similarly, viewing pictures exhibiting compression (telephoto) distortion from a greater distance, thus narrowing the angle of view of the presentation, reduces the effect. In both cases, at some critical distance, the apparent distortion disappears completely.
[edit] Examples
Below, a series of three photos shows the same scene shot from the same position with three different lenses: a normal lens, a wide-angle lens, and a telephoto lens. When viewed at a normal viewing distance, the first photo is the truest representation of the scene. Notice in the second photo that the widened angle has made the structure appear farther away than it actually is, but because the relationship between its size and that of the trees behind it has remained unchanged it has also made the trees seem farther away from it than they actually are. In the third photo, because of the narrower angle of view of the telephoto lens, not only does the metal structure seem closer than it actually is, but the trees in the background also seem closer to it. If the photographer had actually approached the structure, then its image would have expanded to fill the view much more quickly than the image of the trees expanded. The difference is most apparent between the second and third photos, in which the distance between the structure and the trees appears entirely different.
An example of how angle of view affects perspective distortion. The photos above were taken by a 35 mm still camera at a constant distance from the subject with a 50 mm lens, a 28 mm lens and a 70 mm lens.
Below, a series of four photos shows an object framed as nearly the same as possible with four different lenses. As a result of the different angle of view of each lens, the photographer moved closer to the object with each photo. Note that the angle of view changes significantly (compare the background in each photo), and the distance between objects appears greater with each succeeding image. In the fourth image at the lower right, taken with the widest lens, the building behind the object appears much further away than in reality.
Photos taken using a 35 mm camera with a 100 mm, a 70 mm, a 50 mm, and a 28 mm lens, at different distances from the subject.
The process described above has bearing on the in-camera special effect known as the dolly zoom, in which a zoom lens zooms out at the same time as the camera moves toward the subject, in such a way as to keep the subject the same size in the frame while the background "changes size" relative to the subject. This effect was made popular in the films Vertigo and Jaws. Another example of this can be seen in the first Lord of the Rings film, just before the Black Rider comes down the road. The dolly zoom is used to create a compression effect in the road.
[edit] Artistic uses
Although perspective distortion is often annoying when unintended, it is also intentionally used for artistic purposes. Extension (wide angle) distortion is often implemented to emphasize some element of the scene by making it appear larger and spatially removed from the other elements. Compression (telephoto) distortion is often used to give the appearance of compressed distance between distant objects, such as buildings or automobiles in order to convey a feeling of congestion.
[edit] Examples
The photo below of the gondolas is an illustration of the use of extension distortion, and the photo of the buildings below it is an example of compression distortion.
[edit] See also
[edit] Notes
- ^ A distance approximately equal to the diagonal of the picture. This appears to produce the perception of normal perspective when a normal lens was used to take the picture. See, Ira Current, Best Viewing Distance for Photographers, PSA Journal, Sept., 1990 at http://findarticles.com/p/articles/mi_m1306/is_n9_v56/ai_8851922 Retrieved January 22, 2008. A "normal lens" is one having a focal length that approximates the diagonal of the image frame of the camera.
