| 摘要 |
PROBLEM TO BE SOLVED: To obtain various satisfactory aberrations even in the case of use for reading an approximately unmagnified image while achieving a wide field angle, a high capacity, and compactness by constituting a second lens so that its refractive index distribution may satisfy a specific condition formula. SOLUTION: This lens system consists of a first double-convex lens L1 , a second double-concave lens L2 having the refractive index distribution in the optical-axis direction, and a third positive meniscus lens L3 having the concave directed to the object side in order from the object side. The refractive index of the second lens L2 is reduced from the object side toward the image side, and a differenceΔn between the value of the refractive index at the surface apex on the object side and that on the image side satisfies condition formula 1:0 <=Δn<=0.1. Further, when a point on an optical axis X in the second lens L2 where the refractive index is n0 is taken as a reference point, the refractive index n(z) of the second lens L2 is expressed by a condition formula II where (a) is the distance from the reference point to the surface apex on the object side of the second lens L2 and (z) is the distance in the optical-axis direction from the surface apex on the object side of the second lens L2 .
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