An automatic focus detecting device for a camera including a self-scanning type photoelectric conversion element array. The array is operated in two modes, a brightness detecting mode and a focus detecting mode. In the brightness detecting mode, the individual light detecting elements are connected in parallel with one another and to a brightness detection module. Once the brightness detection has been made, the scanning frequency of the array is set in accordance with the brightness value detected. The photodiode elements are then coupled in serial fashion to a focus detecting module. The position of the lens is varied until the focus detecting module indicates that the proper focalization position has been reached.