A distortion is the alteration of the original shape (or other characteristic) of an object, image, sound, or other form of information or representation. Distortion can be wanted or unwanted by the artist. Distortion is usually unwanted when it concerns physical degradation of a work. However, it is more commonly referred to in terms of perspective, where it is employed to create realistic representations of space in two-dimensional works of art.
Perspective Projection Distortion
Perspective projection distortion is the inevitable misrepresentation of three-dimensional space when drawn or "projected" onto a two-dimensional surface. It is impossible to accurately depict three-dimensional reality on a two-dimensional plane. However, there are several constructs available that allow for seemingly accurate representation. The most common of these is perspective projection. Perspective projection can be used to mirror how the eye sees by making use of one or more vanishing points.
Giotto, Lamentation (The Mourning of Christ), 1305–1306
Giotto is one of the most notable pre-Renaissance artists to recognize distortion on two-dimensional planes.
Foreshortening
Foreshortening is the visual effect or optical illusion that causes an object or distance to appear shorter than it actually is because it is angled toward the viewer. Although foreshortening is an important element in art where visual perspective is being depicted, foreshortening occurs in other types of two-dimensional representations of three-dimensional scenes, such as oblique parallel projection drawings.
The physiological basis of visual foreshortening was undefined until the year 1000 when the Arabian mathematician and philosopher, Alhazen, in his Perspectiva, first explained that light projects conically into the eye. A method for presenting foreshortened geometry systematically onto a plane surface was unknown for another 300 years. The artist Giotto may have been the first to recognize that the image beheld by the eye is distorted: to the eye, parallel lines appear to intersect (like the distant edges of a path or road), whereas in "undistorted" nature, they do not. In many of Giotto's paintings, perspective is employed to achieve various distortion effects.
Foreshortening
This painting illustrates Melozzo da Forlì's usage of upward foreshortening in his frescoes at The Basilica della Santa Casa.
Distortion in Photography
In photography, the projection mechanism is light reflected from an object. To execute a drawing using perspective projection, projectors emanate from all points of an object and intersect at a station point. These projectors intersect with an imaginary plane of projection and an image is created on the plane by the points of intersection. The resulting image on the projection plane reproduces the image of the object as it is beheld from the station point.
Radial distortion can usually be classified as one of two main types: barrel distortion and pincushion distortion. Barrel distortion occurs when image magnification decreases with distance from the optical axis. The apparent effect is that of an image which has been mapped around a sphere (or barrel). Fisheye lenses, which take hemispherical views, utilize this type of distortion as a way to map an infinitely wide object plane into a finite image area.
On the other hand, in pincushion distortion, the image magnification increases with the distance from the optical axis. The visible effect is that lines that do not go through the center of the image are bowed inwards, towards the center of the image, like a pincushion. A certain amount of pincushion distortion is often found with visual optical instruments (i.e., binoculars), where it serves to eliminate the globe effect.
Cylindrical perspective is a form of distortion caused by fisheye and panoramic lenses, which reproduce straight horizontal lines above and below the lens axis level as curved, while reproducing straight horizontal lines on lens axis level as straight. This is also a common feature of wide-angle anamorphic lenses of less than 40mm focal length in cinematography. Essentially it is just barrel distortion, but only in the horizontal plane. It is an artifact of the squeezing process that anamorphic lenses do to fit widescreen images onto standard-width film.