| 摘要 |
PROBLEM TO BE SOLVED: To provide an athermal lens system which has excellent achromatic performance and where the temperature fluctuation of a focal position is reduced, and an optical device including the lens system. SOLUTION: This athermal lens system is provided with a 1st lens element L1 consisting of 1st material, a 2nd lens element L2 consisting of 2nd material and a 3rd lens element L3 consisting of 3rd material, and the 1st, the 2nd and the 3rd lens elements L1, L2 and L3 are arranged nearly in tight-contact with each other. The sign (positive) of the refractive powerϕof the entire athermal lens system is the same as the sign (positive) of the refractive powerϕ1 of the 1st lens element L1 and the sign (positive) of the refractive powerϕ3 of the 3rd lens element L3, and opposite to the sign (negative) of the refractive powerϕ2 of the 2nd lens element L2. Furthermore, the lens system satisfies a specified conditional expression.
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