Focusing Systems

Figure 4-7.–Focusing scale. 302.268 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. Focusing Systems Accurate focusing and framing are essential to good pictures, and modern cameras have many devices to help you get good focusing and framing results. Because of the principles of depth of field, simple cameras are manufactured without any way of adjusting focus. The lenses of these simple cameras are prefocused at the hyperfocal distance. Remember from chapter 1, that the hyperfocal distance for a lens is determined by the focal length and the aperture. That allows “point and shoot,” ID, and passport cameras to produce pictures where everything from about one half of the hyperfocal distance through infinity are acceptably sharp. Focusing is accomplished by adjusting the distance from the lens to the film. It does not matter which of the two is actually moved, the lens or the film. With hand-held cameras the lens is moved in and out. Usually on large-copy cameras, the camera back (film plane) is moved toward or away from the lens, That is because the distance from the photographer to the lens board is usually too great to focus through the ground glass. No matter what system you use to focus the camera, there must be a means for you to determine when the image is in focus. Some cameras have autofocusing systems. Most camera systems used by Navy personnel are focused manually. Focusing Scale This is the simplest type of focusing system. It uses a scale of distances that indicates the distance where the focus is set. Primarily, these scales are engraved on the lens barrel. To use the focusing scale, you can measure the camera-to-subject distance, but, in most cases, you must estimate the camera-to-subject distance. This distance is then set to the focus index mark on the lens (fig. 4-7). Scale focusing can be useful when you anticipate quick action but do not have sufficient time to focus the camera. When using scale focusing, a small f/stop is helpful so you can rely on depth of field to provide an acceptably sharp image. Ground Glass Focusing With some cameras, focusing is done by viewing the image on a glass screen, called a ground glass. The image formed by a view camera is projected directly to the ground glass for viewing and focusing. Accurate focusing can be achieved using a ground glass. There is a drawback to this type of focusing. Because of the texture of the ground glass, very fine detail of the image is difficult to distinguish. That results in some leeway in focusing. Additionally, when you work too long at focusing the image, your eye will adjust and accept an image that is less than sharp. For this reason, it is helpful to place a magnifying loop directly on the ground glass. That helps in focusing quickly and accurately. A ground glass focusing system shows directly the image that will appear on the film. The image size and depth of field records on the film the same as it appears on the ground glass. Ground glass focusing systems are commonly found on copy cameras and view cameras. The image on the ground glass appears upside down and backwards. Reflex Focusing A reflex focusing system also uses a ground glass or focusing screen; however, instead of the image being 4-6