This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Distorted Perspectives. Few photographers realize the disadvantages accompanying the use of a lens having a short focal length. The two illustrations (Illustrations Nos. 88 and 89) are designed to show the results which follow the use of a lens of too short focus. The extreme cases have been chosen to more clearly illustrate the principle, but it clearly demonstrates the effect of a lens including too wide an angle, or, in other words, being of too short focal length. Quite often the photographer not only fails to please, but actually loses orders through this very fault in work submitted to the public. The sitter may not know why the fault exists, but he knows that it does exist, and does not like it.
Illustration No. 90 Why a Short Focus Lens Distorts.
914. The diagram (Illustration No. 90) shows why a short focus lens distorts. If the lines E D and F A represent the rays of light from the surface of a spherical body to a long focus lens, it would be seen that light from approximately one-half the surface of the object represented by arc A D reaches the lens. If the lens is of a shorter focus, necessitating that it be placed at G, less than one-half the surface, or the arc C A, will be included in the image, while at the position H only the arc B A will be seen. At the same time the image has been growing larger as we approach the object, so that now the small arc B A is spread over an area several times greater than the semi-circle D A included by the long focus lens.
915. The photograph represented in Illustration No. 88 shows normal perspective and was made with a long focus lens, the distance from the lens to the figure being 18 feet. In the distorted photograph (Illustration No. 89) the lens was one of short focus, and the distance from the figure to the lens was 18 inches. The result in this photograph shows the extreme distortion of the features, completely changing the expression and causing a flattening of the lighting.
Depth Of Focus. While depth of focus in any lens depends upon the focal length and relative aperture, it will be found that the modern anastigmat type of portrait lenses possesses depth of focus in as great a degree as any other lens of similar focal length and aperture. With a given aperture, and at ft given distance, the shorter the focal length of a lens the greater its depth of focus. It should always be remembered, however, that the closer the lens is to the sitter the less will be the depth obtained in the picture; so that the long focus lens shows more depth for large pictures.
917. Depth of focus is also obtained by stopping down the lens. In testing a portrait lens for depth of focus, the above conditions should always be borne in mind, and the photographer should not expect a lens to do more in this respect than the physical laws, which control the formation of all images by lenses, will permit. Illustration No. 91 shows a portrait made by Dudley Hoyt, with a Bausch & Lomb Portrait lens, f/4 of 16 inch focal length.