An automatic focus detecting device for a camera in which a contrast signal is produced which is made independent of variations and brightness of the object being viewed and of vibrations of the camera by utilizing an out-of-focus image on an average illumination detecting photoelectric element. The average illumination detecting photoelectric element is disposed in a plane perpendicular to the optical axis of an optical image of the camera. A pulse generator produces an output clock pulse signal having a frequency which is controlled by the output signal from the average illumination detecting photoelectric element. A self-scanning type photoelectric element array is scanned with this variable frequency clock pulse train wherein the frequency of the clock pulses is increased when the brightness of the object being viewed increases to thereby increase the dynamic range of the device.