Abstract: A focus detector has a photoelectric position sensor which is disposed in a proper focal plane of light beams passing through a pupil splitter having two light transmitting sections which are controllable to be made transparent alternately in an image forming optical system. Output currents from two electrodes of the photoelectric position sensor are processed to derive a signal representing the center of gravity of an image on the position sensor.The signal is compared with the control signal for making the two light transmitting sections transparent alternately for determining an in focus or an out of focus condition and a front focus or a rear focus based on magnitude and direction of change of the signal, respectively.

Abstract: A focusing device for optical systems wherein, in order to make it possible to use an image photographing element and to focus an image without vibrating a body, on a light intercepting plate to be rotated in a plane intersecting at right angles with an optical axis around the optical axis in or near an exit pupil or a position conjugate with the exit pupil of an optical system, light intercepting parts and non-light intercepting parts are so arranged as to alternately vary the relative positions of the light intercepting parts and non-light intercepting parts appearing within the pupil whenever the light intercepting plate makes 1/4 rotation. The light intercepting plate is rotated either continuously or intermittently by 1/4 rotation. The light intercepting parts and non-light intercepting parts appearing within the pupil are alternately varied vertically or horizontally within the visual field.

Abstract: A telephoto lens system is disclosed comprising first, second and third positive meniscus lens components each having the convex surface on the object side, a fourth negative meniscus lens component having the convex surface on the object side, a fifth positive or negative cemented meniscus lens component having the convex surface on the image side and a sixth positive lens component. This telephoto lens system has an f-number of about F/2, a telephoto ratio of about 1.1 and a high performance and is compact.