Perspective distortion (photography)

foreshortened. At "very" small values (not shown, as the camera would lie inside the object), light rays are nearly perpendicular, resulting in lines that normally appear perpendicular seeming to converge upon a single point.

In photography and cinematography, perspective distortion describes one of two phenomena – the appearance of a part of the subject as abnormally large, relative to the rest of the scene, or an apparent lack of distance between objects in the foreground and those behind them.

Influencing factors

Perspective distortion is influenced by only two factors: the distance of the subject matter from the camera and the distance at which the photograph of the subject is viewed.

Camera to subject distance

When photographs are viewed "at the ordinary viewing distance" [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.] , the distance between the subject and the camera 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 requires the camera be moved back from the subject, which results in considerably less perspective distortion. 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 a good portrait lens. Conversely, using lenses with much longer focal lengths for portraits results in a relative flattening of facial features, which also may be objectionable to the viewer.

Wide-angle perspective distortion

This variety gets its name from the fact that it is often produced by the use of wide-angle lenses, though the lens is not the cause of the distortion. It is caused by an object in the scene being much closer to the camera than the remainder of the objects in the scene. The shorter the distance between the camera and the object, the larger the apparent size of that object relative to the remainder of the scene, and hence the greater the perspective distortion.

Telephoto perspective distortion

This variant is so named because it often is produced through use of long focus or telephoto lenses. Again, however, the lens doesn't cause the distortion. Rather, it results from of all subjects in the photo being at a great distance from the camera when a long focus or telephoto lens is used to isolate them from their surroundings. Here, the perspective appears to be compressed. In extreme cases, the foreground and background subjects seem to be piled on top of each other.

Examples

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 film Jaws. An another example of this can be seen in the first movies, just before the Black Rider comes down the road. The dolly zoom is used to create a compression effect in the road.

Photograph viewing distance

Photographs are ordinarily viewed at a distance of from 12" to 18" (30 to 46cm). When viewed at this distance, the distortion effects created by the camera to subject distance are apparent. However, if one views pictures exhibiting so-called "wide angle" distortion at a closer distance, then the phenomenon abates. Similarly, viewing pictures exhibiting the so-called "telephoto" distortion from a greater distance reduces the effect. In both cases, at some critical distance, the apparent distortion disappears completely.

Artistic uses

Although perspective distortion is often annoying when unintended, it is also intentionally used for artistic purposes. The "wide angle" distortion variety is often implemented to emphasize some element of the scene by making it appear larger and spatially removed from the other elements. The "telephoto" variety of 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.

Examples

The photo below of the gondolas is an illustration of the use of "wide angle" distortion, and the photo of the buildings below it is an example of the "telephoto" variety.

ee also

Perspective Control (PC) Lens

Notes

External links

* [http://www.kevinwilley.com/l3_topic04.htm Compressing distance and altering perspective]