| 摘要 |
The image forming optical system includes, in order from the object side, a meniscus, a first lens having positive refracting power with a convex surface directed toward the object side, an aperture stop, a meniscus, a second lens having negative refracting power with a concave surface directed toward the object side, a third lens having positive refracting power with a convex surface directed toward the image side, and a biconvex, fourth lens having positive refracting power. The fourth lens has at least one aspherical surface and satisfies the condition: <?in-line-formulae description="In-line Formulae" end="lead"?>1.0<phim/phip<50.0<?in-line-formulae description="In-line Formulae" end="tail"?> where phim and phip represent power of the fourth lens at a position of the maximum light height and at the paraxial position, respectively. Also, the following condition is satisfied: <?in-line-formulae description="In-line Formulae" end="lead"?>0.3<f1/f<2.0<?in-line-formulae description="In-line Formulae" end="tail"?> where f1 and f represent the focal lengths of the first lens and of the entire image forming optical system, respectively.
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