In the following table Mr. S. A. Warburton has calculated the exposure necessary with every stop from pi to f/iroo compared with the unit stop of the "uniform system" of the Pho tographic Society of Great Britain. The figures which are underlined show in the first column what f/a must be in order to increase the exposure in geometrical ratio from 1/4, the intermediate numbers showing the uniform system number for any other aperture.
Mr. T. R. Dallmeyer has also introduced a system of numbering stops in which the figure obtained by dividing the focal length by the aperture is squared and divided by io.
It is not absolutely necessary that the aperture on the diaphragm shall be cir cular. Many other forms haven been devised. Fig. 159 is a diaphragm much used by the writer in hot climates when the distant mountainous background was so actinic that it was almost impossible to obtain correct exposure of background and foreground together. By using a stop of this kind the actinicity of the sky and background was reduced. A similar effect is obtained with Sutton's sky diaphragm (Figs. 16o, 161 and 162).
The object of this is more clearly shown in Fig. 162. The largest amount of light entering the lens is from the foreground, whilst the sky receives the least.
Fig. 163 is the form of aperture recommended by Fisck for stereoscopic diaphragms.
Within the last few years a new kind of diaphragm, termed the Iris diaphragm has hePrt devised. It consists of a number of thin pieces of metal fastened to a ring in the lens mount. By turning the ring backwards or forwards the aperture of the diaphragm may be enlarged or diminished at will. A pointer on the outside of the ring indicates the different apertures of the diaphragm. Loose diaphragms are very apt to get mislaid or lost. They should be rivetted together, as shown in Fig. 156 Another form is the rotating diaphragm. In this the apertures are
cut upon a circular plate, which is centered, so that its revolution may bring either aperture exactly central with the lens, a small catch indicating to the operator when it is in its proper position. This kind of diaphragm is generally fitted to wide-angle and view lenses.
Each cross line varies in length from the adjacent one by of an inch.
To use: Lay the stop flat on this scale, and select the cross line which is of the same length as the greatest diameter of the opening; read this off, by means of the figures, which will be the measurements in of an inch. The equivalent focal length of lens, divided by this measurement of the stop opening, will give the fraction expressing the ratio of aperture to focal length. The rapidity of different lenses, or of the same lens with different stops, is proportional to the squares of these ratios.
Diffusion of ray of light when it does not arrive direct but is spread about, is termed diffused light. The light of the atelier is usually diffused with ground glass or translucent material. Diffused light in the camera resulting in fog is that light which does not come direct from the lens on to the plate.
Diffusion of passing of one gas into the space occupied by another. The diffusion of gases takes place at a rapidity that is inversely proportional to the square roots of their densities or specific gravities.
Diffusion of two liquids capable of mix ing are placed in contact with each other they will gradually diffuse one into the other.
Diffusion of the focus of the image is spread out it is termed diffused. Dallmeyer constructed a lens in which the back portion of the combination could be moved backwards to diffuse the focus, and give a hazy, soft appearance to the portrait.