1. A portable electronic device with 3D image capture capability, comprising: a first image capture module and a second image capture module; a subject distance estimator, estimating a subject distance; and an image difference control mechanism, adjusting a distance between the first and second image capture modules based on the subject distance by moving at least one of the first and second image capture modules, wherein; the first and second image capture modules capture a first image and a second image, respectively, to form a 3D image after the image difference control mechanism adjusts the distance between the first and second image capture modules; and the image difference control mechanism comprises: a mechanical structure, on which the at least one of the first and second image capture modules which is moved slides; an electric motor, coupled to the mechanical structure to drive the sliding of the at least one of the first and second image capture modules which is moved; an electric motor controller, controlling the electric motor based on the subject distance; and a storage unit storing a look-up table to be checked by the electric motor controller based on the subject distance and, by the look-up table, the shorter subject distance corresponds to a more positive value to drive the electric motor to shorten the distance between the first and second image capture modules.