A method for controlling an autoexposure function of a camera to a gray level setting, the method including establishing a critical exposure time of an image sensing device of the camera, wherein the critical exposure time of the image sensing device of the camera is an exposure time at which a first sensed gray level of the image sensing device of the camera is disposed at a target gray level, the sensed gray level is disposed in a trend of decreasing sensed gray level values; exposing the image sensing device of the camera to light to obtain a second sensed gray level and comparing the second sensed gray level at the critical exposure time to the target gray level, if the second sensed gray level is disposed at least at the target gray level, limiting the exposure of the image sensing device of the camera to the critical exposure time.