Direct-Vision Range Finder Focusing

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. Direct-Vision Range Finder Focusing Cameras that use direct-vision range finder focusing produce a double image in the viewfinder until the subject is in focus on the film plane. This system has a coupled range finder optical device that is linked to the focusing ring. To focus a direct-vision coincidence or split-image range finder camera, you must align two separate images of the subject. When looking through the camera viewfinder, you see a pale or tinted area in the center of the viewing window. This area shows the double image. To set the correct focus, you aim the camera so the subject you want in sharpest focus is in the pale area. You then turn the lens focus ring, or camera 4-7