| 摘要 |
PURPOSE:To decrease the effect of aberration and focus of a lens and also to improve the detecting sensitivity to a position shift margin and out of focus of a photodetector by vignetting an image forming pattern to an information recording medium on the photodetector at focusing. CONSTITUTION:The photodetector arranged near the image forming position to the information recording medium at focusing is provided. Let the focus in regarding the optical system tying the information storage medium 51 and its image forming position as one lens be (f). Let a distance from the image forming position at focusing to the photodetector 70 be DELTA, a distance from a principal point of an objective 66 near the recording medium to a condensed point be F, and the radius of shooting pupil be (r). Let the image forming magnification on the photodetector 70 be (m), and the permissible shift of the information storage medium from the position of the condensed point formed by the condensing means be deltac. The photodetecting section of the photodetector is set to have an area larger than a circle in Equation I . Thus, the effect of the aberration and focus is reduced and the detecting sensitivity to the position shift margin of the photodetector and out of focus is improved. |
