| 摘要 |
PURPOSE:To stabilize operation while maintaining precision by widening a dead zone area, and narrowing down the dead zone area after a focusing state is detected continuously by plural times. CONSTITUTION:When a subject 5 is at distance l2 from an image forming plane 2 and reflected light from a projection spot image P is photodetected in areas 6A and 6B on a photosensor with the same quantity of light, a photodetecting element 6 obtains the difference VA-VB=0 between integral values VA and VB of the outputs of the areas 6A and 6B, and light from a projecting element 3 is passed through an optical path b1 and reflected irregularly by the subject 5 to form an image on the element 6. When the subject 5 moves to at distance l1 while a lens group 1 is assumed to be at an in-focus position, a rear-focus state is entered and the image-forming position of the light on the element 3 shift toward the area 6B, so that the VA-VB is not 0. In this case, the elements 3 and 6 and lens group 1 are moved associatively corresponding to the extent of movement of the subject 5 to form the image on the image forming surface 2 sharply. Therefore, the element 3 moves to 3', the boundary line 6 of the areas on the element 6 moves to 6', and the lens group 1 moves to 1' to obtain a projection optical path a1 and a reflection optical path a2, performing focusing operation so that VA- VB=0. |
