A method for moving an optical device relative to an image sensor to focus an image includes receiving a current image acquired by the image sensor when the optical device is at a current position relative to the image sensor, obtaining a current contrast value of the current image and a current contrast variation rate at the current position, determining whether the current contrast variation rate meets a first threshold criterion, moving the optical device from the current position to a next position in accordance with the determination result, and repeating the aforementioned operations until a next contrast variation rate for a next image meets a second threshold criterion. The next image is acquired when the optical device is at the next position.