| 摘要 |
PURPOSE:To facilitate setting of control center for focus control based upon the just focus state of a recording light beam by a focus servomechanism, by providing an objective unit which includes an objective and a driving means for focus control that drives the objective. CONSTITUTION:A command signal S9 is supplied from the 3rd output terminal O3 of a central processing circuit 19 to a driving circuit 43 and a stepping motor 44 moves a photodetector 13. When the photodetector 13 moves, return beams of a light beam for detection which is incident equally to two photodetecting elements so far and returned from a recording medium 1 are not incident equally to both photodetecting elements any more. Consequently, the difference signal S5 from a subtracting circuit 20 varies in level and a driving current (i) based upon the difference signal S5 flows through the driving means 6 for focus control to drive the objective 5, which is moved to the position where the return beams of the light beam for detection from the recording medium 1 are incident equally to the two photodetecting elements of a photodetector 13. |
