There are two ways for cameras to measure the distance to an object: they can fire a beam of infrared light at it and measure the time it takes to return or they can look at contrast differences in a small area of the object and adjust the lens until you maximise the difference. The former system was common in early compact digital cameras and may still be found in many camera-phones. The latter has always been popular in DSLRs and is becoming increasingly common in digicams, particularly the more advanced models. Many modern cameras combine both systems.
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