When you take a picture, the camera causes light reflected from the subject to be imaged on light-sensitive material. The camera controls this action in several ways. The first control is focus. Cameras have components to show what part of the scene will be recorded in sharp focus on the film. For example, some cameras use a coincidence or split-image range finder, and others use a focusing screen or ground glass.

The second camera control is the lens aperture. This control is located next to the focusing ring on most cameras. As discussed in chapter 1, the aperture affects both focus and exposure.

The third control is shutter speed. The shutter controls the length of time light is admitted to the film. Shutter speed also has an effect on the way movement is recorded on film.

Focusing involves adjusting the distance between

the lens and the focal plane, or film plane, when photographing subjects at various camera-to-subject distances. When a camera lens is focused on a subject point, all light rays from that point, and only that point, are brought to sharp focus at the film plane. When about 600 or more feet from the camera, the subject is considered to be at infinity. A subject at infinity is so far from the camera that rays of light reflected to the lens from the subject are considered parallel. When a camera is focused on a subject at infinity, the distance between the optical center of the lens and the film plane (lens-to-film distance) is equal to the lens focal length. At this point the lens is closest to the film plane. As the camera-to-subject distance decreases, the lens-to-film distance must be increased to bring the subject into focus.

When you are taking a picture of only one subject, focusing is simple; however, when you want to include several subjects at different distances from the camera in the same picture and have them all in sharp focus, it becomes more complicated. Unless the subject is distant scenery with nothing in the foreground, there is always one object that is closer to the camera than another. Then you must decide what part of the scene is to appear in sharp focus. In simple cases, such as a sailor standing against a plain background, the decision is simple-focus on the sailor. In more complex cases, when subjects both close and far from the camera must be in sharp focus, you should focus about one third of the distance into the scene. In other words, focus about one third of the distance between the closest and farthest subject you want in sharp focus. This is known as the depth of field.

The way you focus the camera will depend on what part of the picture is most important and its purpose; for example, the pictures a civil engineer needs of a building at a naval air station is altogether different from the pictures a visitor to the air station wants to take home. The engineer needs pictures that show a maximum amount of detail throughout the scene. The visitor, on the other hand, is more interested in pictures that bring back pleasant memories. The requirements of the picture determine what you should focus on. The engineer needs to have everything in the picture in sharp focus. You might accomplish this as follows: Measure the distance to the nearest point of the picture and the distance to the farthest part of the scene. Then consult the depth-of-field scale on the camera lens to focus on a point between these two distances. Now, when the lens is stopped down to a small aperture, the depth of field is increased. Both near and far points will appear in focus. In photographing the scene for a visitor, you may want to emphasize only the entrance way to the building, rather than concentrating on getting everything in the picture in sharp focus.