1. A method to calculate a contrast value of an image, and the method comprising using a lens to form an image on an image sensing member, where an area of the image is A; taking a retrieving area from the image, where the retrieving area is B; calculating an absolute value of brightness difference of each of pair of neighboring pixels in the retrieving area; and determining a specific value by a distribution of the absolute values and setting a contrast value of the image equal to the specific value.