A portable electronic-device adapted to photograph an object including a first camera module, a second camera module, a processing unit and a control unit. The first camera module obtains a first reference image according to a first predetermined focal length. The second camera module obtains a second reference image according to a second predetermined focal length. The processing unit determines the photographed focal length according to the first reference image and the second reference image. The control unit controls the first camera module or the second camera module to photograph the object according to the photographed focal length. The first camera module and the second camera module face toward a direction corresponding to the object.