In an electronic device, an image point A on an image of an object is selected. A spectral confocal sensor is controlled to move to a position above a measuring point A′ on the object, where the measuring point A′ corresponds to the image point A, and a Z-coordinate of the measuring point A′ is computed using the spectral confocal sensor. A focal position of the measuring point A′ is computed according to the Z-coordinate of the measuring point A′, and a CCD lens is controlled to move to the focal position. The Z-coordinate of the measuring point A′ is stored into a storage unit of the electronic device.