1. An image processing method, comprising: receiving an input of a focal length of a region of interest (ROI); estimating at least one of a first region having a depth value less than the focal length and a second region having a depth value greater than the focal length, based on a depth value of a depth image; and performing out-focusing on the at least one of the first and second regions, by adjusting a color value of the at least one of the first and second regions based on a difference between the focal length and a depth value of a calculation pixel.