Automatic focusing apparatus for camera

Claim 1

An automatic focusing system for a camera, comprising: a light emitting unit which emits a distance measuring light beam; a condenser lens which condenses the reflective light among the light beam projected from the light emitting unit and reflected by an object; a light-spot detector disposed at the focal point of the condenser lens and capable of producing first and second photoelectric currents according to the position of the focused spot of said reflective light on the light receiving surface thereof and providing output signals corresponding to the first and second photoelectric currents, respectively; an arithmetic unit which processes the output signals of said light-spot detector through addition and subtraction; a first variable gain amplifier which amplifies an output signal of said arithmetic unit corresponding to the sum of the output signals of said light-spot detector in response to a first control signal; a second variable gain amplifier which amplifies an output signal of said arithmetic unit corresponding to the difference between the output signals of said light-spot detector in response to the first control signal; first and second rectifying units which rectify the output signals of said first and second variable gain amplifiers, respectively; first and second integrators which receives the output signals of said first and second reclifying units, respectively, and provide integrated voltage signals corresponding to the output signals of said first and second rectifying units, respectively; a level decision unit which detects the level of the output signal of said first integrator; a lens encoder which detects the existing focusing position of a picture taking lens; a comparator which compares the output signal of said second integrator and an output signal of said lens encoder; a lens driving unit which drives the picture taking lens according to the output signal of said comparator to an optimum focusing position; an error detector which receives a switching signal and said output signal of said first rectifying unit, and produces a second control signal for controlling the luminous flux density of said light emitting unit and said first control signal for controlling the respective gains of said first and second variable gain amplifiers, wherein the picture taking lens is driven by said lens driving unit so as to be stopped at an optimum focusing position until the output of said lens encoder coincides with a value corresponding to an output signal of said second integrator at a moment when said level decision unit decides that the output of said first integrator is equal to a fixed value.