| 摘要 |
PURPOSE:To perform focus controlling automatically by passing object luminous flux through the optical system having astignatism and coma aberration and focusing the same with astigmatism and coma aberration in the optical system of a video disc apparatus. CONSTITUTION:The astignatism of the light image of a focus controlling luminous flux 42 on a video disc face 49 is DELTAl/alpha if the magnification of an objective lens 48 is let to be alpha. The focus controlling luminous flux 42 reflected by the video disc face 49 passes through the objective lens 48 and tracking mirror 47 and is again image near relay lens focal plane 46. This luminous flux further passes through a relay lens 45 and becomes the nearly parallel luminous flux having the sectional shape resembling to that of the light image on the relay lens focal plane 46, i.e., the luminous flux having an extremely large astigmatism. This luminous flux is then focused on a focus detecting element 58 through a half mirror 44 and focusing lens 57. Here, the sectional shape of the luminous flux incident to the focus detecting element 58 changes according to the change of the distance between the objective lens 48 and video disc face 49. |
