| 摘要 |
PURPOSE: To maintain excellent image forming performance even when a device has a short path length in glass and a small number of lens elements and not only an initial state, but also lighting conditions and environment change. CONSTITUTION: A refraction type projection optical system is composed of a 1st negative lens group G1, a 2nd positive lens group G2, a 3rd negative lens group G3, a 4th lens group G4, and a 5th positive group G5 in order from the side of a 1st surface A. The effective diameter of the lens faces or the outer diameter of the lenses of the projection optical system become monotonously larger in the 1st lens group G1, tend to become less in the 2nd lens group G2, tend to become larger in the 3rd lens group G3, and tend to become monotonously less in the 5th lens group G5 from the side of the 1st surface A to the side of a 2nd surface B. At least one of the lenses is so held as to have either its position or its attitude adjusted and the numerical aperture of the projection optical system on the side of the 2nd surface B is >=0.8.
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