An imaging device includes a capturing module with at least one lens for capturing an image from a scene, a driving module for driving the at least one lens to different focusing positions using different driving steps, a flat-scene judging module, and a step-judging module. The flat-scene judging module divides the image into a central area and a plurality of peripheral areas, and determines whether the captured scene is a flat scene according to the divided image. The step-judging module determines whether the driving steps corresponding to a maximum focusing value of the central area of the image are same as driving steps corresponding to respective maximum focusing values of the peripheral areas of image, and changes the driving steps corresponding to the maximum focusing value of the central area to the driving steps corresponding to the greatest one of the maximum focusing values of the peripheral areas.