An exemplary portable electronic device includes a main body, camera modules, a storage unit for storing images, and an image processing unit. The camera modules are accommodated in the main body and exposed the ambient environment. The camera modules have different focal lengths from each other, and are configured for simultaneously picking up images of an object. The image processing unit is electrically connected with the camera modules. The image processing unit is configured for comparing the modulation transfer function (MTF) values of the images, and transmitting data of the image having the greatest MTF value to the storage unit.