| 摘要 |
A focussing error detecting apparatus comprises a compound lens and a photodetector, in which the compound lens has first and second lens regions having different back focal lengths separated by a boundary plane which includes the optical axis of light incident on the compound lens. The photodetector has its photodetecting face positioned on the optical axis of the light between the focal points of the two lens regions, and includes first and second photodetecting elements for receiving light passed through the first lens region at its outer and inner sections with respect to the boundary plane, respectively, and third and fourth photodetecting elements for receiving light passed through the second lens region at its outer and inner sections respectively. A focussing error signal is produced from the difference between the sum of signals from the first and fourth photodetecting elements and the sum of signals from the second and third photodetecting elements.
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