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Table 1-2.Amount of light, f/stop, and Exposure Time Relationship
Circle of Confusion

Photography (Basic) - Introduction to photography and other graphic techniques
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f/stops Functions f/stops  have  three  functions: 1. They act as a partial control of exposure (the other  exposure  control  is  the  shutter). 2.   They  help  control  depth  of  field. 3.  They  allow  the  photographer  to  adjust  the aperture to the point of best definition of the lens, sometimes  called  the  optimum  or  critical  aperture. Each of these functions is discussed in this chapter. Focusing A lens, at a given focus setting, provides a sharp image of an object at only one distance in front of it. However, when the distance between the focal plane and the lens can be adjusted, the lens can be made to form sharp images of objects located at differing distances in front of it. Therefore, to get a sharp image of a subject at a given distance, you must adjust the lens to the appropriate   distance   from   the   film   plane.   This adjustment  is  known  as  focusing. In focusing a camera lens, the nearer the subject is to the lens, the farther behind the lens the image is formed. For close subjects, the lens must be moved away from the film plane to focus the image; and the farther away the subject is from the lens, the closer to the lens the film plane must be (fig. 1-25). INFINITY FOCUS.–When the lens is focused on an object so distant that the light rays reflected from it are  parallel,  these  rays  converge  (after  refraction  by  the lens) at the point of principal focus. The point of principal focus is on the principal focal plane; that is, at a distance of one focal length behind the lens. Therefore, the lens is said to be on infinity focus. When the distant object is moved nearer to the lens or the lens is moved closer to the object, the distance between the focal plane and the lens must be increased to keep the image in sharp focus. When the distance between the lens and focal plane is not extended as the object is moved nearer to the lens, the image of the object becomes blurred or out of focus. The closer the lens is to the object it is focused upon, the larger the image becomes until the distance between the lens and the focal plane is extended to twice the focal length of the lens. At this distance, the image and the object focused upon are the same size. Therefore, the size of an image formed by a lens is dependent upon two factors: the distance from the lens to the object focused upon and the focal length of the lens. Figure 1-25.–Subject distance and focus. 1-22







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