| 摘要 |
A beam splitter is provided in the optical path of an objective of a camera or the like to divide the light beam into two light beams. At least in one of the divided light beams is inserted a wedge type prism to shift the image viewed from therebehind when the image is not focused precisely on the prism. At optically conjugate positions in the divided light beams are located photodetectors to receive light. The outputs of the photodetectors are connected with an A.C. component detecting circuit by way of an alternating switching circuit so that an A.C. component of the output of the switching circuit may be detected when the outputs of the photodetectors are different from each other. When the image is focused precisely on the prism, the outputs of the photodetectors become equal to each other and the output of the alternating switching circuit does not have an A.C. component. A servomotor for controlling the position of the objective is connected with the A.C. component detecting circuit by way of an electric control circuit so that the objective is stopped at the position where the A.C. component of the output of the alternate switching circuit is zero.
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