Focusing system
Focusing means setting the distance of the subject to have a sharp picture.
The simpler cameras (for example disposable cameras) have no focus setting at all: this is fixed focusing.
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. (Chronologically they appeared in that order.)
From the beginning of the 1980s the cameras began to incorporate an auto focusing mechanism: the camera determines the distance to the subject automatically. There are two main types of autofocus: active autofocus, where an infrared light 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 focusing systems are detailed in separate pages:
