In traditional camera designs, focusing is done manually, the photographer checking the focus either visually (through the viewfinder of an SLR camera) or by means of a rangefinder. Most modern cameras, however, have automatic focusing: a small central area of the view is measured, the information is fed to a central processing unit (CPU), and this in turn activates a servo-motor that moves the lens into focus.
The details of the operation are more complex than this, but the principle is straightforward. There are some differences between cameras in how the focus is measured, but the usual method involves checking the contrast between the edges of objects seen through the lens (in an out-of-focus image these edges are blurred, so the contrast is less than when the image is sharp).
In most scenes, some parts will be nearer than others to the camera, and this has important implications on focusing. With the lens focused sharply on one object, such as a face, things nearer to and further from the camera will be out of focus to some extent. The degree of difference in sharpness between nearer and further objects depends on a number of factors, including the focal length of the lens, the aperture being used, and the depth of the scene itself. The depth of sharpness achieved in an image is known as depth of field.
