A method for determining an illumination source with optimized depth of focus includes the following steps. First, a simulated optimal correlation and a simulated defocus correlation of each illumination source are provided. Second, an optimal peak is determined, a defocus peak is determined, and an optimal correlation slope and a defocus correlation slope are determined. An optimal correlation ratio and a peak variation are calculated. A correlation variation is calculated from the optimal correlation ratio and the defocus correlation ratio. Next, a weighted variation is determined from the peak variation and the correlation variation. An illumination source of a lowest weighted variation among a plurality of the illumination sources is determined to be an illumination source with optimized depth of focus.