| 摘要 |
PROBLEM TO BE SOLVED: To provide a retrofocus lens excellent in optical performance though it is compact and low-priced while securing sufficient back focus. SOLUTION: The lens system has a 1st lens in meniscus shape turning a convex surface to an object side, which has negative refractive power, a 2nd lens whose both lens surfaces are concave and which has positive refractive power, an aperture diaphragm, a 3rd lens whose both lens surfaces are concave and which has negative refractive power, a 4th lens which has positive refractive power, a 5th lens whose both lens surfaces are convex and which has positive refractive power in order from the object side, and satisfies conditions; 0.5<DB/f<0.8, nd2<1.7, and 0.8<bf/f<1.0 assuming that a distance between the 2nd and the 3rd lenses is DB, the refractive index of the material of the 2nd lens is nd2, the back focus thereof is bf and the focal distance of an entire system is f. COPYRIGHT: (C)2004,JPO
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