| 摘要 |
In an image-forming optical system in which an astigmatism may occur, an astigmatism correcting element disposed between an image-forming lens (20) and an image-forming surface (12) in order to correct the astigmatism is constructed such that it can correct the T image surface toward both plus and minus directions with respect to the S image surface, while easily correcting the astigmatism even when there is fluctuation in the generated astigmatism. The astigmatism correcting element comprises a prism unit (22) in which a pair of wedge-like prisms (22a, 22b) having an identical apex angle are oppositely placed in reverse to each other with a predetermined air space (d) therebetween. Also, this prism unit (22) is disposed such that its optical axis (X1) can be tilted with respect to a main ray (X2) of a bundle of rays subjected to astigmatism correction. The angle of inclination of the optical axis (X1) of the prism unit (22) may be appropriately set so as to nullify the astigmatism of the bundle of rays subjected to astigmatism correction or to intentionally provide a desired astigmatism in this bundle of rays, regardless of whether the main ray (X2) in the bundle of rays subjected to astigmatism correction is on the optical axis (X0) of the image-forming optical system or not.
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