A method for operating an electronic device, which is easy on eyes, is provided. The electronic device includes a display device including a light-emitting device and a light-receiving device, and a lens. The method includes a step of displaying an image for focus adjustment of a user's eye; a step of detecting a spot diameter of first light reflected by the user's eye; a step of moving the lens and detecting a spot diameter of second light reflected by the user's eye; a step of determining whether the spot diameter of the second light is smaller than the spot diameter of the first light; a step of further moving the lens and detecting a spot diameter of the third light reflected by the user's eye in the case where the spot diameter of the second light is smaller than the spot diameter of the first light; a step of determining whether the spot diameter of the third light is smaller than the spot diameter of the second light; and a step of moving the lens to a position at which the spot diameter of the first light has been detected, in the case where the spot diameter of the second light is larger than the spot diameter of the first light.