Abstract: A photo-electric pulse generating unit for monitoring the focus position of a camera objective lens driven in response to a range detection system and operative to generate pulses on rotation of a lens coupled encoder wheel having angularly spaced apertures movable in relation to a detector unit housing the photo-electric pulse generating unit. The unit mounts a pulse generating photo-transistor in alignment with a light source such that the photo-transistor is spaced behind an opaque wall opening to "tunnel" light from a light source upon passing through an aperture in the encoder wheel. Light diffusing surface irregularities are provided to prevent impingement of reflected or ambient light on the photo-transistor.