| 摘要 |
PURPOSE:To detect a focus error simply and surely. CONSTITUTION:A returning light collected at a lens system 25 is detected by a first and a second photodetecting elements 30 and 28. An optical path length- changing means 31 formed of a transparent plate member is arranged on an optical path from the lens system 25 to the first photodetecting element 30. A collective position of the returning light obtained by the lens system 25 when the transparent plate member 31 is removed from on the optical pat is formed at a separated position from the lens system 25. |
