AUTOMATIC FOCUS ADJUSTING DEVICE FOR CAMERA

摘要

PURPOSE:To make the quantity of extension of a lens proportional to the difference between two light outputs which vary reciprocally according to the quantity and position of photodetection of reflected spot light from a subject by multiplying those light outputs by different coefficients and performing control so that the sum of both light output is constant. CONSTITUTION:When infrared light which is pulse-modulated by a light emission control circuit 10 is emitted by an infrared-light emitting diode 1, the reflected light from the subject 3 is photodetected by an optical position detector 5 to obtain output currents I1 and I2. Photodetecting circuits 6 and 7 outputs voltages V1 and V2 corresponding to the currents I1 and I2 and a subtracting circuit 8 calculates V1-V2 while an adding circuit 9 calculates V1+aV2. The light emission control circuit 10 controls the quantity of projection light from the infrared-light emitting diode 1 so that V1+aV2 is constant. Here, when A and (a) are selected so that an equation (6) and an equation (1) become equal, V1-V2 and the quantity (x) of lens extension coincides with or correspond to each other. Namely, the output (V1-V2) of a subtracting circuit 3 is the quantity of the lens extension.