Focusing mechanism - sharpening the image

The lens on a fixed focus camera, as the term fixed focus implies, is locked in place at the factory and cannot be moved. The lens is pre-focused to be sharpest at about 3 to 5 meters (10 to 15 feet). Its aperture is made small enough so that "reasonable" sharpness extends from roughly 2 meters to infinity. For the photographer using such a camera "focusing" consists merely of staying at least 2 meters from the subject.

While this arrangement may be satisfactory for someone who doesn't wish to get involved with technical matters and who takes only an occasional point and shoot type of picture, it won't do for the more serious photographer.

One of the earliest focusing mechanisms was a flexible leather bellows placed between the lens (mounted on a lens board) and the film holder. The bellows stretched or contracted as a small gear moved the lens board back and forth along a rack (a toothed metal strip) until the subject appeared its sharpest on a ground glass viewing screen. This rack and pinion focusing method is excellent. It is often applied to twin lens reflex and other medium format cameras as well.

Rack and pinion focusing is not suited to smaller camera, because their shorter focal length lenses require finer adjustment. Most often an arrangement is used which is much like close fitting tubes, one sliding inside the other. The inner section supports the lens while the outer section is attached to the camera body. Then, when a focusing ring on the lens mount is turned, a helical screw moves the lens back and forth as a unit. Precise focus with even very short lenses can be attained, because a fairly large movement of the focusing ring produces only a small in and out movement of the lens. Macro lenses often use a double helical system to further extend the lens tube and allow still closer focusing.

Another arrangement focuses a lens by moving just the front element on a helical screw while the rest of the lens remains stationary. This is often the least expensive way to achieve a focusing lens, but it does entail both optical and mechanical drawbacks.

Whatever the method of focusing a lens, a photographer has to know when the image is sharp. Many cameras use a ground glass viewing screen. Some provide a direct view. Still others reflect the image off a mirror before it hits the ground glass. Most quality cameras also employ a pentaprism above the mirror so that when the image is viewed it is both upright and correct from side to side.

Non reflex cameras of all sizes often use a split image or coincidence type rangefinder to focus their lenses accurately. In addition, there are some cameras that have a rangefinder as well as ground glass focusing.
Nowadays, all the cameras are equipped with lenses that focus automatically in several ways.
 

More about Camera Lens Features


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