In one method, a beam of infrared (IR) light is projected from the camera.
The beam scans the scene and reflects back from any object it meets.
A sensor on the camera measures the angle at which the beam returns, translates this into a subject distance, and sets the lens accordingly.
However, this method is prone to interference from, say, an intervening pane of glass, which reflects the beam prematurely.
The other method uses a pattern of light from the camera or flash unit, which directly assists the focus detectors to assess the image itself.
Viewfinder systems Four viewfinder systems (to enable you to frame and focus an image before recording it) can be found in digital cameras.
The simplest uses an LCD (liquid crystal display) screen to present a low-resolution image of the subject.
Most cameras offer in addition a direct-vision viewfinder.
This is a small lens (separate from the image-taking lens) through which you view the subject.
Having this allows you to turn off the LCD and so save battery power, but since you are not viewing the subject through the image-taking lens, subject framing is prone to inaccuracy.
The best system is the single lens reflex (SLR) arrangement, identical to that in a conventional camera.
It gives you an accurate view of the subject and makes focusing very easy.
An alternative is what appears to be an SLR system, but the traditional focusing screen is replaced with an LCD screen viewed through the eyepiece.
This screen is sufficient for framing but resolution is not high enough for precise focusing.