How autofocus cameras work?

Improvement in technology has led to development of lenses that can be easily focused to get a sharper picture. With the autofocus technology, a chip automates the whole focusing. The camera lens adjusts itself by moving back and forth and when the recording chip perceives a sharp image, the movement ceases, allowing you to click the picture.

The autofocus technology uses a controlled system of motors and sensors to focus on a point or an area. While some cameras using the technology have a single sensor others high quality ones can have a range of them. The latest SLR (single-lens reflex) cameras use optical sensors, which can monitor the light metering. Through-the-lens autofocusing is more precise and speedier.

Active autofocus

This system measures the distance between the subject and the lens regardless the optical system. The optical system is adjusted once the measurement is done to focus correctly. To measure the distance, infrared light waves and ultrasonic sound frequencies are used. In case of infrared detection, the light wave is used for triangulating the distance while the ultrasonic detection works according to the principle of reflection. The time taken for the sound to reflect back gives the measurement of the distance.

Camera models:
• Polaroid (ultrasonic autofocus)
• Nikon 35TiQD (infrared autofocus)

Passive autofocus

This system of focusing passively analyses the image falling on the optical chip, thereby determining the correct focus necessary. Unlike infrared and ultrasonic technology used in active autofocusing, the passive autofocus uses natural light for determining the distance. However, when the light conditions are upto the mark, an infrared assist beam does the job. This type of autofocusing can be achieved through contrast measurement or phase detection.

Contrast measurement

A sensor field is used to measure the contrast, facilitated by the lens. The difference between intensity of adjacent pixels increases with better image focusing. The adjustment is done until the maximum contrast is achieved. Focus tracking is not possible since it is not known whether the subject is in back focus or front focus. However, this technology is more accurate than the phase detection technology, which we will discuss next. Most DSLRs, video cameras and common digital cameras use the contrast measurement technology.

Phase detection

Phase detection is something that is achieved when the incoming light rays are divided into pairs and the two images that are produced are then compared. This is the most common technology used in SLR cameras. Beam splitter system is used to produce the images in the autofocus sensor. Light rays from opposite sides are diverted into the Autofocus sensor thereby creating a rangefinder. The two images are compared with respect to phase difference, and the focus position is adjusted.

Camera models:
Cannon EOS-1V, Nikon D2X, Cannon EOS-1D.

Comparison of the two focus systems

Since glass reflects sound and light waves the active autofocus system will not work while photographing subjects near windows. With the passive AF technology, this will however, not be a problem. The active system also fails when close objects are to be focused. While these are the disadvantages with active autofocusing, passive AF depends heavily on natural light.

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