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Focusing means setting a lens appropriate to the distance of the subject so as to have a sharp picture.
The simpler cameras (for example disposable cameras) provide no way to alter the focus: this is fixed focus. Some cameras have auxiliary lenses to introduce into the light path to change the focus to a few fixed distances. A few folding cameras focus by moving the lens plate back and forth.
On most cameras, though, you focus by setting the distance on a ring around the lens. How do you know that distance, and how will you focus the image? On the simpler old cameras, you had no indication and had to guess it: this is scale focusing or guess focusing. The more advanced cameras had some kind of focusing help, the main types are the ground glass back, the rangefinder and the reflex finder (in chronological order of appearance).
Normally the entire set of lens elements is moved together when the focusing barrel of lens is turned. (For a small number of cameras, the same effect is achieved by fixing the lens relative to the body and using a knob on the back of the body to move the film plane.) In simpler designs, only the front element is moved - Front Element Focusing. In recent, complex designs, one set of elements may be moved while another is not, or different elements may be moved in different ways.
From the beginning of the 1980s cameras began to incorporate an autofocusing mechanism: the camera determines the distance to the subject automatically. There are two main types of autofocus: active autofocus, where an infrared light (or a sonar pulse) is emitted by the camera and reflected by the subject, and passive autofocus, where the image transmitted by the lens is analysed for its sharpness by a detector inside the camera. Active autofocus is used by the simpler point and shoot cameras, and passive autofocus is used by the autofocus SLR cameras.
The different designs for focus are detailed in separate pages: