| 摘要 |
An objective optical system includes, in sequence from the object side, a first group having positive refractive power and including a plano-convex lens with the convex surface facing the image side; a second group having positive refractive power and including a lens whose extreme-object-side lens surface is convex facing the object side; a third group having negative refractive power and including a lens whose extreme-image-side lens surface is concave facing the image side; a fourth group having positive refractive power and including a lens disposed on the extreme object side, whose image-side lens surface is convex facing the image side and a lens disposed on the extreme image side, whose object-side lens surface is convex facing the object side; and a fifth group having positive refractive power and including a combined lens by joining a convex lens and a concave lens, the joined surface having negative refractive power.
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