An electronic device comprises a computing module and a storing module. The computing module receives a first image having a first region with two spots, determining whether a first interval between the spots of the first region is larger than a threshold, computing the first image with a parameter of a parameter set for obtaining a second image having a second region corresponding to the first region with two spots if the first interval is larger than the threshold, and determining whether a second interval between the spots of the second region is larger than the threshold. The storing module records the parameter, if the second interval is smaller than or equal to the threshold.