the mechanism that can cause slightly different lens openings at the same setting, depending on whether the f-stop is approached by stopping down from larger apertures or opening up from smaller ones. The f-stop relates exposure to the effective diameter of the lens, but disregards certain other factors, primarily the efficiency of the lens in its actual transmission of light. Since lenses of many elements are less efficient than those of few elementsbecause of reflection of light at each surface and the optical density of the glassesattempts have been made to develop a scale that indicates the actual transmission of a lens. A scale of lit-stops� has sometimes been substituted for f-stops to indicate light transmission. These values are seldom seen today, except in some lenses for cinematography, primarily because the efficiency of lenses has been greatly increased by lens coating techniques. The t-stop values, while fine for determining exposure, also distort other mathematical values that relate directly to the true f-stop, such as depth of field and hyperfocal distance.
FOCUS AND DEPTH OF FIELD As the distance from the camera to the subject changes, the distance behind the lens where the image is sharply focused also changes. The image of a nearby subject is sharply focused farther behind the lens than that of a distant subject. Focusing the lens involves adjusting its distance to the film to produce a sharp image of the subject. Focusing with a small camera is usually accomplished by rotating a ring on the lens barrel; with a view camera, the length of the bellows is adjusted by moving the front or rear standard. We can achieve critical focus for only one plane in front of the camera, and all objects in this plane will be sharp. In ad-
dition, there will be an area just in front of and behind this plane that ,will appear reasonably sharp (according to the standards of sharpness required for the particular photograph and the degree of enlargement of the negative) . This total region of adequate focus represents the depth of field. It is a property of lenses that as we reduce the aperture used for exposure, the depth of field increases. Thus, if it is important in a photograph to have areas close to the camera appear approximately as sharp as more distant ones, we select a small aperture. There are two other factors that affect the depth of field: the focal length of the lens (change to a shorter focal length lens if you need more depth of field) and the subject distance (move away from the subject to increase depth of field) . These three factors, aperture, lens focal length, and subject distance, give us considerable flexibility in managing depth of field. In addition, the lens and film plane adjustments of a view camera/1 while they do not actually change the basic depth of field, do permit us to align the plane of sharp focus to coincide with the most important plane of the subject. The factors affecting depth of field are governed by the following principles : (1) The depth of field doubles if the f-number is doubled (e.g., from f/8 to f/16). (2) If you double the subject distance, the depth of field increases by four times triple the distance, the depth of field increases by nine times (depth of field is proportional to the square of the distance). (3) If you reduce the focal length by one half, the depth of field increases by four times (depth of field is inversely proportional to the square of the focal length). We must remember that the depth of field relates to an acceptable degree of sharpnessj in actuality, only the plane focused upon is truly sharp. Acceptable sharpness is also affected by the degree of enlargement of the negative and the distance from which the final print is viewed.