A focusing method includes the steps of calculating a diameter difference between a first circle of confusion diameter and a second circle of confusion diameter which correspond respectively to first image data and second image data associated respectively with light energy of a first spectrum and light energy of a second spectrum, generating a control signal associated with a target image distance according to an initial image distance, the diameter difference, and a lookup table, driving movement of a lens module so as to change a distance between the lens module and an image sensor from the initial image distance into the target image distance according to the control signal.