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 1-26. Focusing for one object.

Figure 1-27. Image on film in front of and in back of the point of sharp focus.

FOCUSING FOR ONE OBJECT. Focusing is done essentially to obtain the proper distance between the lens and the film. When light rays come from a far object and pass through a lens, they form a sharp image close to the lens. When light rays come from a near object, they form an image farther away from the lens. This means that the lens must be focused on either the far or the near object, depending on which one the photographer wants to have in sharp focus. When a sharp image of the near object is desired, the lens should be focused by moving it farther away from the film. When you want a sharp image of the far object, move the lens closer to the film (fig. 1-26).

CIRCLE OF CONFUSION. A picture is basically an accumulation of many points that are exact images of points composing the subject. After light strikes a subject, it is reflected from many points on the subject. A camera lens redirects these reflected rays into corresponding points on the film. Each of these points is reproduced by the lens as a circle. When the circle is smaller than l/100 inch, it appears as a sharp point to the eye. When the circle is larger than 1/100 inch, the eye sees it as a circle, and the image is blurred or out of focus. Each out-of-focus circle on the film is called a circle of confusion and can be visualized as the cross section of a cone of a light ray (fig. 1-27).

Figure 1-28. Depth of field.

When a lens is focused on an object at a certain distance, other objects, both closer and farther than the focus distance, form larger circles of confusion. When the film is placed at a point corresponding to the lens focus distance, a clear image is produced (fig. 1-28). When the film is nearer or farther away from the lens than the corresponding lens focus distance, the image becomes blurred because of the larger circles of confusion caused by the intersection of light rays either in front of, or behind, the film plane.

Another factor affecting the circle of confusion is lens aperture. Decreasing a lens opening narrows the light rays passed by the lens. The narrower these rays, the smaller the circles of confusion when the image is not in perfect focus. In practice, this means that a small lens opening is used to record, as clearly as possible, several objects at varying distances. Even when the rays from some objects do not intersect perfectly at the film plane, the circles of confusion ahead or behind the film are negligible and still appear as a sharp image.

The size of the permissible circle of confusion depends on the film format size and the manner in which the film will be used. Experience has shown that the permissible circle of confusion should not exceed about 1/1000 of the focal length of the lens. This is normal for the film size. The generally accepted permissible circle of confusion diameters are given in table 1-3.

The minimum circle of confusion of most lenses is small. Thus the focal plane can be moved slightly and yet retain an acceptable sharp image. However, as the distance of the movement is increased, the circle of confusion becomes greater and the image becomes less

Table 1-3. Permissible Circle of Confusion Is Dependent on Film Size

Figure 1-29. Depth of focus.

sharp. Consequently, the distance that the focal plane can be moved forward or backward from the plane of sharp focus and continue to produce an image of acceptable sharpness is termed the depth of focus. This depth is always within the camera (fig. 1-29).