This study aims at developing a focusing method for an image with multiple focal distances. A dynamic focus region (DFR) is defined to find the object of interest to be focused on. The DFR is built from the edge fragments detected using several narrow ROIs across the vision screen. The proposed method was applied to the camera module assembly. The inspected images usually have multiple areas with different focal distances, due to the different thicknesses of the components. An assembly machine was constructed to verify the DFR for the experiment. The distance between the lens mount and vision camera was adjusted to find the focus position by means of the sharpness and DFR. The result shows that the desired object is brought into focus and the proposed method can be applied to the camera module assembly process.