| 摘要 |
PURPOSE:To prevent malfunction at automatic focusing by correcting deficiency of dynamic range of a detecting circuit to a degree without hindrance in the range of normal operation. CONSTITUTION:If a focus voltage is lowered, the dynamic range of a detection circuit 4 surpassed it and the detection circuit 4 cannot sometimes detect the voltage. In order to prevent it, the 2nd threshold level L2 is set to a level slightly higher than the minimum detecting level of the detecting circuit 4 by a threshold value circuit 11, and if the focus voltage is lower than the 2nd threshold value level L2, a discriminating circuit 12 switches a switch circuit 14 so as to increase the focus voltage thereby increasing the amplification factor of an amplifier circuit 10. Thus, the focus voltage is kept in the dynamic range of the detection circuit 4 at all times regardless of the state of an object O, thereby preventing malfunction of the automatic focus circuit. |
