An Autofocus assist beam or AF assist beam is a lamp (usually red) which is incorporated into photographic cameras and flash units. This beam usually projects a regular pattern on to the subject which is recognized by the camera's AF system and used for better autofocusing in low light situations.
Some cameras, for example the Canon EOS 20D, do not incorporate a dedicated AF assist beam but can use its internal flash as a stroboscopic flash which lightens the scene with several continuous flashes. This helps the AF to work more precisely, but can be very irritating to the subjects. In some cases an external flash with AF assist beam can be purchased for better results.
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