2 thoughts on “Q&A: How does a camera lens autofocus?”

1. Basically it is controlled by a tiny on-board IC chip, as well as some sophisticated mechanical devices.

   Some cameras can even be told to keep certain colours in continuous focus, something sports photographers love.

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2. Well this need a LooonG explanation… but lets make it as short as possible:

   There are two kinds of focusing:
   1. Active Focusing (as used by small point and shoot cameras)
   2. Passive Focusing ( as used by expensive DSLR cameras)

   1. Active Focusing

   You must have read about sound navigation ranging (SONAR), which is used by underwater submarine to bounce a sound wave off the subject. This focusing works in the same manner, however, an infrared signal is used instead of sound waves. The camera sends out invisible infrared light energy in pulses when in focus mode. The subject reflects the invisible infrared light back to the camera, and the camera’s microprocessor computes the time difference between the time the outbound infrared light pulses are sent and the inbound infrared pulses are received. Using this difference, the microprocessor circuit tells the focus motor which way to move the lens and how far to move it. This focus process repeats over and over while the camera user presses the shutter release button down half-way. It is great for subjects within 20 feet (6 m) or so of the camera.

   To use infrared focusing effectively, be sure the emitter and the sensor have a clear path to and from your subject. The infrared beam can bounce off of something in front of the subject rather than making it to the subject (like, bars/net at a zoo cage). If the subject is not exactly in the middle, the beam can go right past the subject and bounce off an undesired subject in the distance, so be sure the subject is centered.

   Infrared sensing can have problems from black subjects as the surface may absorb the outbound infrared beam. Also, an open flame (like, birthday cake candles) or very bright surface can confuse the infrared sensor.

   The advantage of an active autofocus system is that it works very well in the dark, making flash photography much easier.

   On any camera using an infrared system, you can see both the infrared emitter and the receiver on the front of the camera, normally near the viewfinder.

   2. Passive Focusing

   Passive auto-focus determines the distance to the subject by computer analysis of the image itself. A typical autofocus sensor is a charge-coupled device (CCD) that provides input to algorithms that compute the contrast of the actual picture elements. The CCD is typically a single strip of 100 or 200 pixels. Light from the scene hits this strip and the microprocessor looks at the values from each pixel. The microprocessor then searches for the point where there is maximum intensity difference between adjacent pixels- that’s the point of best focus. The camera then drives the lens motor back and forth searching for the best focus.

   Passive autofocus must have light and image contrast in order to focus well. There is no distance-to-subject limitation with passive autofocus like there is with the infrared beam of an active autofocus system. Passive autofocus also works fine through a Zoo cage, since the system “sees” the subject through the window just like our eyes do.

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