| 摘要 |
PROBLEM TO BE SOLVED: To provide an optical system having a diffraction grating composed so that a good image quality can be obtained by suppressing the occurrence of a flare on a non-lens surface, and to provide a photographing device and an observation device which has the optical system. SOLUTION: In the optical system having a diffraction surface consisting of the diffraction grating of a blaze type having rotationally symmetrical form to an optical axis, the flare on the non-lens surface is suppressed by setting the ratio of the whole area of the non-lens surface and the average of the projection area to a plane vertical to the optical axis of the diffraction surface included in the optical system, to be in a prescribed range.
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