A method and an apparatus for operating an electronic device is provided. A first image is obtained at a first position of a focus lens of a camera module of the electronic device. The first image is output through a display. A second image is obtained at a second position of the focus lens while the first image is being output. A defocus amount of the focus lens at the second position is calculated based on the first image and the second image. An on-focus region is detected based on the calculated defocus amount.