1. A method for image taking brightness control, comprising: acquiring a preview image frame, and determining a brightness level n of each pixel in the preview image frame; dividing the preview image frame into M class areas according to the determined brightness level n of each pixel; calculating a feature weight for each class area in the preview image frame, wherein calculating includes, for any one of the class areas, calculating a detail feature value of each subarea in the class area, counting total detail feature values of all subareas in the class area, calculating a pixel ratio of a number of pixels in the class area to a total number of pixels of the preview image frame, and determining the feature weight of the class area in the preview image frame according to the calculated pixel ratio and the counted total detail feature values; for any one of the class areas, determining a ratio of the feature weight of the class area to the sum of the feature weights of all class areas as a feature weight ratio of the class area; determining an exposure adjustment level required when the preview image frame is taken according to the feature weight ratio of each class area; and adjusting a pre-acquired exposure parameter according to the determined exposure adjustment level, and taking an image according to the adjusted exposure parameter.