1. A sharpness processing method for a digital image, applied to an image capturing device to capture a raw image, the method comprising: loading the raw image; performing an image processing procedure on the raw image to generate a registered image; loading a first sharpness enhancement table corresponding to sharpness attenuation of an outer-edge region of the registered image; loading a second sharpness enhancement table corresponding to sharpness attenuation of a central region of the registered image, and defining a range between the outer-edge region and the central region as a plurality of critical regions; and performing a sharpness processing procedure on the registered image by the image capturing device, comprising: loading a sharpness enhancement weight table, and searching for sharpness enhancement weight values of each of the critical regions respectively corresponding to the first sharpness enhancement table and the second sharpness enhancement table according to the sharpness attenuation distribution of each of the critical regions of the registered image; calculating a sharpness enhanced value of each of the critical regions according to the first sharpness enhancement table, the second sharpness enhancement table, and the sharpness enhancement weight values of each of the critical regions respectively corresponding to the first sharpness enhancement table and the second sharpness enhancement table; and respectively performing a differential image sharpness processing procedure on the corresponding critical region by using the calculated sharpness enhanced values, so as to correct the registered image into the digital image and output the digital image.