| 摘要 |
In a large-aperture lens for low-illuminance imaging, a Gauss type lens configuration is employed, and a negative lens having a predetermined refracting power is inserted in front of the front-side cemented lens, whereby back focus is secured while compactness is attained, without using an aspherical lens. Successively disposed from the object side are a biconvex lens L1, a positive meniscus lens L2 having a convex surface directed onto the object side, a negative lens L3 having a concave surface with a stronger curvature directed onto the image side, a positive meniscus lens L4 (cemented with lens L5) having a convex surface directed onto the object side, a negative meniscus lens L5 having a concave surface directed onto the image side, a biconcave lens L6 (cemented with lens L7) having a surface with a stronger curvature directed onto the object side, biconvex lenses L7 and L8 each having a surface with a stronger curvature directed onto the image side, and a positive meniscus lens L9 having a convex surface directed onto the object side. Assuming that the focal length of the whole system is f and the focal length of the third lens is f3, the conditional expression of 0.35<f/|f3|<0.56 is satisfied.
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