The advance of photography : its history and modern applications (1911)

122 THE ADVANCE OF PHOTOGRAPHY lens, when used in a camera at the same distance from the objects, but it must at the same time be remembered that at a greater distance from the objects the long focus lens will be no more affected than the short focus lens is in its present position, and the resulting photograph, which may be on the same scale, will be a much better one as regards perspective. The required amount of definition can, however, be given without increasing the distance, by simpry adjusting the diaphragm, so as to reduce the aperture of the lens. It must be mentioned here that want of distinctness in a photograph may add to its pictorial value, and a want of definition in one part of the photograph may be lost sight of if the definition of the whole subject is suitably lowered (see diffusion of focus, p. 125). It must be remembered that if we have two lenses of equal focal length and intensity, then the depth of focus must be the same for each. Lenses which are of the same focal length but of different intensity have different depths of focus, the one with the smaller intensity having the greater depth, but of course if the faster lens is stopped down to the same intensity, then they will both have the same depth of focus. While dealing with this subject, it might be of interest to mention the principle upon which the action of the fixed focus cameras depends. For this purpose, without going into any proof of the formula used, let us assume that the distance beyond which all objects may be considered in focus depends upon three things — the focal length of the lens used, the intensity of the lens, and the degree of distinctness which is aimed at. It can be shown that the distance varies directly as the intensity, and also directly as the square of the focal length. With respect to the degree of distinctness, it is generally agreed that good clear results are not to be obtained if the