1. A method, comprising: capturing a first test image without using a flash; capturing a second test image using the flash with a predetermined intensity; obtaining a difference image indicative of a difference between the second test image and the first test image, wherein the difference image comprises a plurality of difference image pixels, each difference image pixel having a pixel value; identifying a reference pixel among the difference image pixels, wherein the first test image comprises a first test pixel corresponding to the reference pixel, and the second test image comprises a second test pixel corresponding to the reference pixel; and computing illumination amount by the flash for subsequent image capturing based at least partly on pixel value of the first test pixel and pixel value of second test pixel, wherein the illumination amount by the flash for the subsequent image capturing is achievable by causing the flash to provide flash intensity by a trigger pulse having a pulse width, wherein when the pulse width is smaller than a predetermined value, the pulse width is increased and the flash intensity is reduced accordingly so as to maintain the illumination amount by the flash.