An image sensor comprising: an image sensing matrix, comprising at least one image sensing unit, for generating at least one image sensing signal according to a sensed image; an analog to digital converter, for converting the image sensing signal to a digital image sensing signal; an adjusting unit, for adjusting the digital image sensing signal to be an adjusted digital image sensing signal according to at least one adjusting parameter and the digital image sensing signal; an operational circuit, for computing at least part of brightness of the sensed image sensed by the image sensing unit according to the adjusted digital image sensing signal to generate at least one operational brightness signal; and a control unit, for adjusting the adjusting parameter, such that brightness information generated based on brightness values, which corresponds to the operational brightness signal, falls in a predetermined range.