| 摘要 |
PROBLEM TO BE SOLVED: To lighten the load of a focusing action and to reduce the deterioration of resolution caused by the fluctuation of an original by making the curvature of the field small and making depth large as an image forming lens constituted of five groups and six lenses. SOLUTION: The 1st positive lens L1 whose object side is convex, the 2nd negative lens L2 whose image side is concave, the 3rd positive meniscus lens L3 whose object side is convex, the 4th positive meniscus lens L4 whose image side is convex, the 5th negative lens L5 whose object side is concave and the 6th positive lens L6 whose image side is convex are arranged in turn from an object side. Besides, the 1st lens L1 and the 2nd lens L2 are cemented. Then, they are constituted so as to satisfy the conditional expressions of 0.51<f34/f<0.72, 0.83<f1/f6<1.17 and 1.17<(ν1/ν3)1/2/ν2<1.62. In the expressions, (f) shows the focal distance of a whole system, (f34) shows the synthetic focal distance of the 3rd and 4th lenses, (f1) and (f6) show the focal distance of the 1st lens and the 6th lens andν1,ν2 andν3 show the Abbe numbers of the materials of the 1st to the 3rd lenses.
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