| 摘要 |
An objective optical system whose outer diameter is reduced, whose overall length is increased, in which chromatic aberrations are well corrected, and which is suitable for in-vivo examination with a high numerical aperture is realized. There is provided an immersion objective optical system comprising, in order from an object side: a first group having positive power; a second group having positive power; a third group having negative power; a fourth group having positive power; and a fifth group having positive power, wherein the first group includes a plano-convex lens L 1 whose convex surface faces an image-plane side, and a surface at the extreme object side is a substantially flat surface; the second group includes a lens L 2 in which a surface at the extreme object side of the second group is a convex surface facing the object side and a lens L 3 in which a surface at the extreme image-plane side of the second group is a concave surface facing the image-plane side; the third group includes a compound lens in which a biconvex lens L 4 and a biconcave lens L 5 , from the object side, are joined; the fourth group includes a positive lens L 6 whose convex surface faces the object side; and the fifth group includes at least one compound lens.
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