| 摘要 |
PURPOSE:To stabilize the detection of an out-of-focus state by providing a photodetector, which is arranged in a position deviated from the image formation position for the in-focus state and has plural detecting cells, and a means which moves a spot in accordance with the deviation of a storage medium and placing one detecting cell on the optical axis at least. CONSTITUTION:The beam of an out-of-focus state detecting system which is divided by a half mirror 10 is taken out asymmetrically with respect to an optical axis 14 by a light shielding plate 13 and is made incident on a photodetector 16 having light detecting cells 16-1 and 16-2. An area 21 of the detector 16 has not an optical detection sensitivity practically. The light receiving face of the photodetector 16 is deviated from the image formation position in the in-focus state. The position of a center point 21' of the area 21 is deviated from an intersection 14' between the optical axis 14 and the detector 16. A width 21'' is so narrowed that the point 14' is placed on the outside of the area 21. In the out-of-focus state, a spot 22 is deviated upward from the center of the light receiving face and is larger than that in the in-focus state. A reflected beam from an object lens 5 is convergent. In this case, the spot 22 is converged once in a position deviated from the point 21'; and when the out-of-focus degree is increased, it is inverted downward. Thus, the out-of-focus state is detected stably. |
