| 摘要 |
PROBLEM TO BE SOLVED: To provide an infrared kens which is used as the image forming lens which can be used in the infrared area of long wavelength being 8-12μm, whose F value us small such as <=1.0 and whose aberration is small. SOLUTION: A 1st lens group G1 constituted of a positive meniscus lens L11 whose convex surface faces an object side, a 2nd negative lens group G2 constituted of a negative concave lens L21 whose concave surface faces the object side, a 3rd positive lens group G3 constituted of a positive meniscus lens L31 whose convex surface faces the object side, and negative meniscus lens whose convex surface faces the object side are all arranged in the above order toward an image side 3 from the object side 2. The 1st, the 2nd and the 3rd lens groups are constituted of the germanium-made lenses. Besides, space D12 between the 1st and the 2nd groups and the focal distance (f1) of the 1st lens group are set so as to satisfy the condition of 0.4<D12/f1<1.
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