An auto focus system comprises an image capturing optical unit, an image capturing camera, an image acquisition and processing engine, an acutance index calculation engine, a motion control engine, a motion controller and a lens driving unit. An auto focus method measures image's acutance index rather than image's contrast value. An auto focus system employing the auto focus method of the present invention can accurately and reliably detect the best focus point and thus preventing auto focus malfunction even under the condition that illumination uniformity changes dramatically.