DISTORTION. When the image formed by a lens on the focussing screen of a camera obscure does not satisfy the conditions imposed by the rules of perspective, it is said to be distorted.
Distortion of the image in the camera obstura is in general pro duced by the excentrical incidence of the oblique pencils. Its effect is to render all straight lines which do not pass through the centre of the picture curvilinear, and also to exaggerate the dimensions of certain objects in the picture, with respect to others, in a way which is opposed to the principles of true perspective. We shall explain how distortion arises when a large view lens is used with a dia phragm in front, as this arrangement offers an illustration of the evil in a very marked degree.
Suppose P Q R to be a triangle plaoed in front of a large view lens / m, having a diapinlagm in front at H. First, let the lens be removed, and the size of the aperture at H reduced indefinitely. Then, images A, B, C, of the points P, Q, R, will be formed on the focussing screen by producing the lines P H, Q H, R H, until they cut it, and the triangle A B C on the focwasing screen will be a correct perspective view of P Q R.
Now introduce the lens / m.
The small oblique excentrical pencil P H, incident at m, is bent out of its course by refraction through the mar gin of the lens, and comes to a focus at a. Similarly, the pencils from Q, R, have their foci at b, c. The image given by the lens is, therefore, the triangle abc. This triangle is smaller than A B C ; but, if the distances A a, B b, Cc, are proportional to 0 A, 0 B, 0 C, the two triangles will be similar figures, and there will be no distortion. But it happens that the decrements A a, B b, C c, are not proportional to the radial lines OA, 0 B, 0 C, but increase more rapidly than these do. There is, therefore, distortion of the image, as shown in the figure.