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

Figure 4-10. Direct-vision range finder focusing.

focus knob, until the double images coincide and only one image is seen (fig. 4-10).

The disadvantages of a direct range finder system are that it does not couple to a large variety of lenses, thus restricting its use to only several different focal-length lenses. Unlike the ground glass and SLR focusing systems, depth of field cannot be determined in the direct-vision range finder system. Everything appears sharp through the viewfinder window.

Autofocus

Most autofocus cameras use the same principle as a direct-vision range finder camera. The autofocus camera determines the subject distance by comparing

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Figure 4-11. Use of selective focus.

the contrast brightness of two images: one reflected from a fixed mirror, the other from a movable mirror. This system works on the theory that the sharpest images have the highest contrast. When maximum contrast is reached, an electronic device converts the contrast brightness information into impulses. These impulses start a motor that moves the lens to the point of sharp focus. This type of autofocus system does not perform effectively when the subject is all one color or does not contain much contrast.

Another type of autofocus camera uses sonar or infrared These systems emit either a sonar or infrared signal to determine subject distance. The distance is determined by the amount of time it takes the transmitted energy to reflect back from the subject to a sensor on the camera. This information is then sent to a motor that moves the lens to the point of sharp focus. The sonar autofocus system has a disadvantage. You cannot photograph subjects through glass. The sonar reflects off the glass and not the subject.