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; determining a specific value by a distribution of the absolute values and setting a contrast value of the image equal to the specific value; analyzing statistically numbers with the same absolute value of brightness difference to obtain the distribution, where the distribution comprises the numbers of same absolute value of brightness difference; accumulating the numbers of same absolute value of brightness difference from a maximum absolute value to a minimum absolute value; and setting the contrast value of the image equal to a first absolute value of the brightness value when an amount of accumulations of the numbers of the same absolute value of brightness difference is equal to or larger than a pre-determined value after accumulating the number of the first absolute value of the brightness value.