AUTOMATIC FOCUSING DEVICE

摘要

PURPOSE:To focus always an objective subject by detecting the edge part of the change of an electrical signal and extracting only a previously fixed part out of the edge detecting signal. CONSTITUTION:An edge detecting circuit 15 detects the edge part of the change of an electrical signal obtained from a pre-amplifier 3. The image of a subject formed an image on an image pickup surface of an image pickup element 2 by a lens 1 is most sharply formed at the time of focusing and its signal change is sharply edged. Thereby, the amplitude of a signal obtained from the edge detecting circuit 15 is maximized similarly to that of a high frequency component and the amplitude is reduced in accordance with defocusing. Since the edge detecting signal is obtained by detecting only the edge part of the change of an output signal from the image pickup element, the time delay of a response between an input and an output or the time expansion of the change of the output signal is not almost generated. Consequently, the signal of the subject other than a passage part can be completely removed by a picture angle gate signal and only the subject in the passage part can be properly focused.