| 摘要 |
PROBLEM TO BE SOLVED: To provide an image forming optical system concurrently satisfying the reduction of cost, high performance and miniaturization. SOLUTION: The image forming optical system is constituted by arranging a positive meniscus lens L1 turning its convex surface to an object side, which functions as a 1st lens, a brightness diaphragm S, a meniscus lens L2 turning its convex surface to an image side, which functions as a 2nd lens, a positive meniscus lens L3 turning its convex surface to the image side, which functions as a 3rd lens, and a negative lens L4 which functions as a 4th lens in order from the object side. At least one surface of the 4th lens L4 is aspherical and the lens L4 satisfies a following conditional expression. -2.0<ϕm/ϕp<0, provided thatϕm is the power of the 4th lens at the position of maximum light beam height, andϕp is the power of the 4th lens in a paraxial state. The 3rd and the 4th lenses L3 and L4 are composed of plastic and satisfy a following conditional expression. 15.0<ν3-ν4<40.0, provided thatν3 is the Abbe number of the 3rd lens andν4 is the Abbe number of the 4th lens. COPYRIGHT: (C)2005,JPO&NCIPI
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