| 摘要 |
PROBLEM TO BE SOLVED: To obtain an achromatic lens system which consists of three lenses of positive, negative and positive lenses, satisfies a predetermined conditional expression to excellently correct color aberration in areas from visible light to infrared light and is light approximately by F5.0. SOLUTION: An achromatic lens system of the present invention is configured by disposing single positive, negative and positive lenses in order from an object side so that the negative lens and at least one of the positive lenses satisfy conditional expressions (1) and (2). Furthermore, in this achromatic lens system, the at least one positive lens satisfies a conditional expression (3). Additionally, this achromatic lens system satisfies conditional expressions (4) and (5). In this case, said conditional expression (1) is ¾θ<SB>nC, t</SB>-θ<SB>pC, t</SB>¾<0.03, the conditional expression (2) is ¾θ<SB>ng, F</SB>-θ<SB>pg, F</SB>¾<0.02 (whereinθ<SB>nC, t</SB>,θ<SB>ng, F</SB>are partial dispersion ratios of the negative lens ((t) rays, (g) rays), andθ<SB>pC, t</SB>,θ<SB>pg, F</SB>are partial dispersion ratios of the positive lenses ((t) rays, (g) rays)). Further, said conditional expression (3) isν<SB>P</SB>>80 (whereinν<SB>P</SB>is Abbe's number of the positive lenses). Moreover, said conditional expression (4) is ¾ϕ<SB>P</SB>/ϕ<SB>T</SB>¾<5.0 and the conditional expression (5) is ¾ϕ<SB>N</SB>/ϕ<SB>T</SB>¾<5.0 (whereinϕ<SB>P</SB>is the sum of inverses of focal distances of both the positive lenses,ϕ<SB>T</SB>is an inverse of a focal distance of the entire system, andϕ<SB>N</SB>is an inverse of a focal distance of the negative lens). COPYRIGHT: (C)2005,JPO&NCIPI
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