Digital camera auto-focus (AF) optical system uses sensor, ultrasonic sound wave, infrared light, phase detection or contrast measurement to focus an image. Some camera optical system may use more than one method for this focusing mechanism. Mechanical movement of the lens is done by an electrical motor, which is powered by a battery. A camera, which also offers manual focusing mode along with auto mode has a option to disengage the motor drive train.

Mainly this mechanism either use active or passive method for focusing. Active mechanism uses sensors, ultrasonic sound waves or infrared light to measure the distance of the object, which is to be focused. Professional cameras may use more than one device to deal with low light condition. This system will fail if we try to focus an image through a glass window pane as the system will be misled by the object (glass) in front of it.

Passive mechanism analyse phase detection or contrast of an image projected on the electronic view finder. Phase detection system uses a beam split-er and analyse the image for its sharpness. Contrast detection system calculates contrast difference between two adjacent pixels and focus an image when maximum measurement is achieved. Assist lamp is used, which throws visible or infra red light to assist contrast detection system, when the ambient light is low.

Depending on the model of a camera, this focusing system may offer continuous focusing of the object, even if the object is moving towards or away from the camera. This feature is very useful in sports and wild life photography.

Whichever system is used for focusing an image automatically, most of the time, it will work well and give a sharp image. For better and faster results, a camera model may use multiple systems, which improves the performance of the focusing mechanism, but at the same time, the cost of the camera is increased.

A lens designed for auto-focus has a very smooth and light weight focusing barrel movement. This is for reducing the load on the electrical motor and to maximize the focusing speed. Be careful with these lenses, as when you are using it on manual mode, focus may shift because of smooth movement of the focusing ring.

Use this facility, which will remove the bounden of focusing and you will be able to concentrate more on the subject, composition and lighting while taking photographs.

Read more about:

Accessories in photography


Auto and manual mode of a camera

Camera lens

Camera support

Depth of field


Flash light


Light attachments

Other photographic accessories


Studio flash lights

Telephoto lens


Types of cameras

Wide angle lens

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