|
|||||||||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Perspective distortion (photography) |
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.
Contents |
Perspective distortion is influenced by only 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.
When photographs are viewed at the ordinary viewing distance1, 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.
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 angle of view of the capture are apparent. However, 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.
Perspective distortion occurs when the apparent distance from the viewer to every object (or part of an object) in an image changes at a constant relative rate.
An object's absolute distance from the viewer is its actual distance away in some unit of linear measure. It's relative distance from the viewer is its distance compared to another object as expressed by a ratio or a percentage.
For example, you could have two identical stone statues one of which is 10 meters in front of you in absolute distance while the other is 1 meter beyond at 11 meters in front of you. The relative distance of the far object to the near object is a ratio of 11/10 or 10% farther away.
If you walk toward the statues until you are only 1 meter away from the nearer one you will be two meters away from the other in absolute distance. However, it will now be twice as far from you as the other in relative distance, a ratio of 2/1 or 100% farther away. The nearer statue will have gotten bigger in your field of vision at a faster rate than the farther statue and the farther statue will appear to be twice as far away. This is normal perspective. No distortion is taking place.
If, on the other hand, you remain 10 meters away from the nearer statue and take a photo with a telephoto lens (or crop and blow up a photo taken with a normal lens, or just look at it through a telescope) so that the statue that is 10 meters away appears as if it were only 1 meter away, the relative distance of the other statue to it will appear unchanged at a ratio of 11/10 or only 10% farther away. In other words, through a telephoto lens, when the nearer statue appears as though it were 1 meter away, the farther statue will appear to be only 1.1 meters (1 meter and 10 centimeters) away. The nearer and farther statues will have gotten bigger in your field of vision at the same rate. Perspective distortion is taking place. Two objects that are actually 1 meter apart appear to be only 10 centimeters apart.
Conversely, if you actually close to within 1 meter from the nearer statue and shoot a photo with a wide-angle lens that makes it appear as though it were 2 meters away, again the relative distance between the two will appear unchanged at a ratio of 2/1 (this time), or 100% farther away. In other words, through a wide-angle lens, when the nearer statue appears as though it were 2 meters away (while it is actually 1 meter away), the farther statue will appear to be 4 meters away. The nearer and farther statues will have gotten smaller in your field of vision at the same rate. Again perspective distortion is taking place. Two objects that are actually 1 meter apart appear to be 2 meters apart.
Because perspective distortion occurs in only one direction, a radius from the viewpoint to every part of the image, it can also appear to change the shape of objects. Each object appears compressed or extended in one direction only.
Below, a series of three photos shows the same scene shot from the same position and angle with three different lenses: a normal lens, a wide-angle lens, and a telephoto lens. The first photo is the truest representation of the scene as it actually appeared. Notice in the second photo that the widened angle has made the metal 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 film Jaws. An another example of this can be seen in the first Lord of the Rings movies, just before the Black Rider comes down the road. The dolly zoom is used to create a compression effect in the road.
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.
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.
 |
 |
 |
 |
 |
 |
 |
 |