Automatic Focusing

DEFINITION:

A system or function which automatically detects and adjusts the focal position of the lens in relation to the distance to object.

DESCRIPTION:

The following are methods to detect distance to object;

1. Triangle method: Point a is observed from points b and c distanced in a predetermined space. With angle A obtained at this time, distance to object R is ascertained.          

    To detect angle A, the following systems are used:      

    (1) Coincidence type: An overlap of two images in the right and left is identified in the form of an electrical signal, and A is detected.

    (2) Light beam system: The strength of the light striking an object and reflected back from the object is converted to an electrical signal, and A is detected.          

    (3) Gravity triangle system: Angle A is detected mechanically by pointing the camera from the horizontal position down to the foot of the object, based on the standard height of a human body.  

  2. Time difference detecting method: A light beam, radiowave, or soundwave is projected against the object, and the distance is obtained from the time taken for the beam or wave to return from the object to the camera.

  3. Focal contrast detecting method: The sharpness of an image at the focus is detected directly. The contrast at the mat plane of a single-lens reflex camera is converted to an electrical signal, and thus, distance is detected.