| 摘要 |
PROBLEM TO BE SOLVED: To set the number of lenses to about 2 to 4 and to excellently compensate aberration by providing a radial type refractive index distribution lens having axially symmetric refractive index distribution in a radial direction in a rear group, and satisfying a specified condition. SOLUTION: This lens system is constituted of a front group consisting of a positive lens and a negative lens, and the rear group consisting of the positive lens. In the rear group, at least one radial type refractive index distribution lens having the axially symmetric refractive index distribution in the radial direction and expressed by an expression Nλ(r)=N0λ+N1λr<2> +N2λr<4> +... is used. Then, the lens system satisfies the condition 1/V1 <1<1/V0 , and -0.5<N1×f<2> <-0.01. Provided that (r) is a distance from an optical axis in the radial direction, Nλ(r) is a refractive index at the point being the distance (r) of wavelengthλ, Niλis the refractive index distribution coefficient of 2i- degree of the wavelengthλ, V0 and Vi are values expressed by V0 =(N0d -1)/(N0 F-N0 C) and Vi =Nid /(Ni F-Ni C), and (f) is the focal distance of the entire lens system. N0d ... and Nid ... are the refractive index and the distribution coefficients of 2i-degree on optical axis with respect to a line (d), a line (F) and a line (C), respectively.
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