1. A photometric calibration method, comprising the steps of: obtaining a target image and a reference image by a light field camera and transforming the reference image into a revised target image corresponding to the target image; calculating, by a processor, a photometric calibration ratio according to a function for correlating the target image and the revised target image by using a numerical algorithm; and dividing, by the processor, an intensity of the target image by the photometric calibration ratio to obtain a distortion-free image corresponding to the target image.