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

Figure 4-7. Focusing scale.

Figure 4-8. Twin-lens reflex camera.

formed directly on the ground glass or focusing screen, the lens forms the image on a mirror that reflects the image to the focusing screen or ground glass.

TWIN-LENS REFLEX. The twin-lens reflex (TLR) system uses a matched set of lenses for focusing and viewing. One lens is the viewing lens; the other is the picture-taking lens. The viewing lens is always wide open. That makes focusing and viewing easy, but depth of field cannot be viewed. Depth of field must be determined by a scale that is provided on the lens or camera body.

An advantage of the twin-lens reflex system is that the image is visible on the focusing screen, before, during, and after exposure. A disadvantage of twin-lens systems is that parallax errors occur. Parallax refers to the difference between the image seen through the viewing lens and the image transmitted to the picture-taking lens (fig. 4-8). For distant subjects the difference is not very great or noticeable; however, when your subject is close to the camera, parallax is much more noticeable. You see a different image area through the viewing lens than what is being transmitted through the picture-taking lens. Some twin-lens reflex cameras have an indicator in the viewing lens, so you can compensate for parallax. Another disadvantage of the twin-lens reflex camera is that it takes practice to

Figure 4-9. Split-image focusing.

follow action and compose the subject. The image seen on the focusing screen is backwards from the actual image. Twin-lens reflex cameras are no longer commonly used in Navy imaging, but they are still around.

SINGLE-LENS REFLEX. Single-lens reflex (SLR) cameras have a focusing and viewing system that shows you the image formed by the picture-taking lens. SLR cameras are designed so the distance between the focusing screen and the lens is exactly the same as the distance between the lens and the film; therefore, whatever appears in focus on the focusing screen will also be recorded in focus on the film. With an SLR camera, there is no parallax error.

Sometimes two small prisms or a split screen is included in the central area of an SLR camera viewing screen. When the image is out of focus, it appears split in this area (fig. 4-9). Some screens have a central grid of minute prisms that produce a shimmering effect when the image is out of focus.

An SLR camera is focused by rotating the focusing ring on the lens until the image seen through the viewfinder is in sharp focus. SLR cameras are the most commonly used camera in Navy imaging today.