to an automatic diaphragm driving coil 17 to obtain a suitable aperture value in fig2 reference numerals 18 designates a brake coil which detects an automatic diaphragm moving speed to obtain a suitable operation speed thereby to stabilize the control operation fig3 shows a waveform 19 of a video signal applied to the analog switch 12 a waveform 20 at the output terminal 13 of the video signal extracting circuit 1 and a waveform 21 of the output of the filter circuit 14 the waveforms shown in fig3 are those of signals which are present when the picture region c is selected in the above described embodiment the filter circuit 14 is an rc circuit type as shown in fig2 its output represents an average value of the extracted video signal however the filter circuit 14 may be replaced by a different processing circuit such as a circuit which outputs the peak value of an extracted video signal if desired the video signal applied to the analog switch 12 must be fixed in black level with the automatic exposure device thus constructed a cameraman can select a video signal corresponding to a desired picture region as follows while observing the scene through the camera monitor the cameraman determines which of the picture regions a b c and d includes the main object of interest and selects the video signal corresponding to.