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.
57. Diaphragms or Stops. The diaphragm or stop in a shutter is usually an opening which can be regulated from outside the lens barrel, making it larger or smaller in proportion to the amount of light it is desired to admit to the plate, and also in proportion to the sharpness of definition that is required on the plate.
58. Waterhouse Stops - Iris Diaphragm. Originally diaphragms were metal plates, each having an opening in the center, of different diameters, ranging in geometrical proportion to the focal length of the lenses to which they belong. They are known as Waterhouse stops, and are inserted in an opening in the lens barrel. The majority of lenses and shutters fitted to the common hand cameras now contain a device known as the Iris Diaphragm, whereby the opening may be decreased or increased in size by the sinble movement of the indicator, B. See Illustrations Nos. 7 and 8. When this lever is turned to any of the figures on the plate at the bottom of the shutter it indicates the proportionate amount of light admitted. The smaller the aperture the greater will be the depth of focus of the lens.
59. Depth of Focus. By depth of focus is meant the power which a lens has of rendering a sharp image upon the ground-glass, of objects situated at varying distances. When using the lens at full opening, it is difficult to obtain at one and the same time a clear and sharp image on the ground-glass of both near and distant objects. To obtain this increased sharpness you must make use of the stops or diaphragms. This gives increased depth of focus.
60. Use of Diaphragms. The diaphragm markings are usually arranged at the lower part of the front of the shutter. (See Illustrations Nos. 6, 7 and 8.) By using the smaller opening, diaphragming down, or "stopping down," as it is commonly termed, a portion of the rays are cut off and a much greater depth of focus secured; but in employing a smaller opening, cutting out the rays of light, the exposure necessary is greatly increased.
Study No. 2. Eventide By G. A. Brandt
61. There are many defects in some of the cheaper grades of lenses, and, aside from giving a greater depth of focus, the use of a smaller stop corrects different aberrations or defects. The more important points to be remembered in using a diaphragm are these:
Second, the smaller the aperture the longer the exposure, the greater the depth of focus and the sharper the image, accompanied by a corresponding loss in relief. When focusing always have your lens wide open (full diaphragm). After obtaining the focus, stop down only enough to give clear detail in the picture.
62. To Focus the Camera. Whatever the type of shutter used, set it at the time exposure and open, using the largest stop or diaphragm. The light will then pass through the lens, permitting you to see the image on the focusing screen or ground-glass. By moving the lens support - i. e., on folding cameras - to and fro on the track, the image on the ground-glass can be brought to a focus, which means clear, sharp definition. With kodaks which are not provided with a ground-glass or focusing screen the focus is obtained by gauging the distance from the camera to the object being photographed, and setting the pointer on the lens support opposite that distance indicated on the focusing scale.
63. Regulating Different Openings According to Exposure. For snap-shot or instantaneous exposures employ the largest diaphragm or opening. In average view work - that is, open landscapes - it is seldom necessary to stop down, i. e., reduce the aperture, smaller than the f/8, or its equivalent U. S. 4. (See following paragraph.) The smallest opening should be used only when extreme sharpness of the whole field is required. For example, when photographing an interior it will be necessary, in order to secure detail, to have objects close to the camera, in the middle distance, and those more remote, equally sharp.
64. The Size of the Stops are numbered in different ways, according to various systems. The two in most use are the "f" and the "U. S." (Uniform System.) The f system refers to the relation of aperature to focal length; while the U. S. is a uniform series of markings based on the same principles. More detailed information regarding these systems is given in the advanced instruction in Volume VI. It may be as well to state here, that on shutters of American manufacture the Uniform System of numbering the stops is universally used, while the diaphragms on anastigmat lenses of foreign manufacture are marked according to the f system.
65. The corresponding values are given in the following series:
66. The exposure with a certain stop is one-half of the next smaller and double that of the one next larger; i. e., if stop f/8 requires an exposure of one second, the next smaller, f/11.3, will need two seconds; while f/5.6, the next larger, will need but one-half second's exposure, and f/4 one-fourth of a second. Or, in the U. S. numbers stop No. 4 requires one second, No. 8 will need two seconds, No. 2 one-half second, and No. 1 one-fourth of a second. The largest opening or the largest working aperture of the lens on the average hand camera is f/8 or U. S. 4. For the beginner, who is apt to become confused by the diaphragm numbers, it is well to remember that the large numbers indicate small openings, and that the smaller the opening the longer the exposure required.
67. Reversible Back. All modern high grade folding plate cameras are supplied with a reversible back. This is to enable the user to make either horizontal or vertical views by merely reversing the back attachment without changing the position of the camera. The back is held in place by means of firm clamps, which are easily released when desired. The ground-glass is attached to the frame of the reversible back, so it is always in correct position.
68. Focusing. Focusing a camera means the act of bringing the image into focus; i. e., the securing of a clear, sharp outline of the image upon the sensitized plate or film. The focus is secured by moving the lens a certain distance from the ground-glass or focusing screen. This distance depends upon the focal length of the lens employed.