A program shutter for automatic exposure control including a forwardly/reversibly rotatable step motor for opening and closing a shutter to effect automatic exposure in the control circuit for controlling the operation of the step motor. The control circuit includes stored predetermined shutter mechanism characteristic values. The reversible stepping motor varies the lens aperture in response to a pulsed lens control signal generated by the control circuit. The corresponding value associated with each pulse is varied to be synchronized with and dependent on the shutter mechanism characteristic values and correspondingly varies the rotational power of the reversible stepping motor to overcome detrimental inherent shutter mechanism characteristics and to increase the response efficiency of the shutter.
A camera having a lens shutter mechanism in accordance with the present invention comprises sectors lying in a lens frame and forming a predetermined exposure aperture, a drive ring for opening and closing the sectors, a high-speed sector opening lever and a low-speed sector opening lever which drive the drive ring through an interlock lever, a plurality of springs having different strengths and constraining the high-speed sector opening lever and low-speed sector opening lever, a sector closing lever for driving the high-speed sector opening lever and low-speed sector opening lever in a direction causing the sectors to close, a mode changing member for changing shutter speeds by selecting whether both or either of the high-speed sector opening lever and low-speed sector opening lever should be driven, a charge cam gear for resetting the shutter mechanism to an initial position after exposure is completed, and a groove bored along the optical axis in a camera body and designed to guide the drive ring along the optical axis when the lens frame collapses and to restrain the drive ring from rotating about the optical axis after the lens frame completes collapsing. Only a minimum number of required members are disposed in a lens frame and the remaining members are arranged in a camera body. This results in a compact camera.
A stepping motor including a rotor having a plurality of magnetic poles, one stator base arranged near the rotor, a pair of magnetic yokes fixed on the stator base and arranged to face the magnetic poles of the rotor, a pair of excitation coils for exciting the pair of magnetic yokes respectively in response to drive pulses which have phase difference, and a pair of stator magnetic polar portions integrally formed with the stator base. The stepping motor is constructed to form magnetic paths together with the number of magnetic turns of the excitation coils without increasing the size of the magnetic yokes, thereby enhancing operations and reliability.
