| 摘要 |
An objective lens having a stop provided most adjacent to the image side comprises, in succession from the object side, a first lens which is a positive meniscus lens having its convex surface facing the object side, a second lens which is a biconcave lens, a third lens which is a biconvex lens, and a fourth lens which is a negative meniscus lens having its convex surface facing the image side and is joined to the third lens and formed into a compositely positive cemented lens. The objective lens satisfies the following conditions: (1) 0.28f< SIGMA d<0.36f (2) 1.58<n2<1.745 (3) n1<n2 (4) 0.24<n3-n4<0.30 (5) 0.6f<f1<1.0f (6) 0.30f<-r6<0.40f (7) f<-r7<2f (8) 10< nu 4- nu 3<25 (9) 2<(d5+d6)/d3<9.9 (10) 0.18f<d1+d5+d6<0.24f (11) 0.28f<r1<0.31f where SIGMA d: the full length of the lens system f: the focal length of the lens system ri: the radius of curvature of the ith surface from the object side di: the spacing of the ith surface from the object side ni: the refractive index of the ith lens from the object side nu i: the Abbe number of the ith lens from the object side f1: the focal length of the first lens.
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