| 摘要 |
PURPOSE:To prevent a photographic lens from being displaced erroneously due to the influence of natural light by matching the focus of the photographic lens according to the level of a range finding signal reflected from a subject. CONSTITUTION:Light emitted from a xenon discharge bulb 6 is converted into an infrared-ray signal through an infrared-ray filter 7, and this infrared-ray signal is radiated toward the subject, so the infrared-ray signal reflected by the subject is converted by the phototransistor 14 of a signal detecting circuit to develop a detection voltage corresponding to the infrared-ray signal by a resistance 15 for detection, thereby comparing this detection voltage with a reference voltage by a comparator 18. The comparator 18 generates an output signal when the detection output rises above the reference voltage to turn on a switching circuit 20. Namely, when the subject is at less than specific distance, an electromagnetic device 21 is powered on by the conduction of the switching circuit 20, so the rotation of the photographic lens is stopped. The photographic lens has the focal length matched with the subject at the close position while inhibited from rotating. |
