| 摘要 |
PROBLEM TO BE SOLVED: To obtain a scanner optical system where chromatic aberration is excellently corrected in a near infrared area by providing a positive front group, a diaphragm and a positive rear group and satisfying specified conditions. SOLUTION: This system is equipped with the positive front group GF, the aperture diaphragm S and the positive rear group GR. The front group GF has a 1st lens component L1 which is positive and a 2nd lens component L2 which is obtained by bonding positive and negative lenses and which is meniscus- shaped and negative, and the rear group GR has a 3rd lens component L3 which is obtained by bonding negative and positive lenses and which is meniscus-shaped and negative and a 4th lens component L4 which is positive. Then, the conditions 1<(fa/fb)/|β|<3, -0.12<Da/ra<-0.01 and 20<ν2p-ν2n are satisfied. In the expressions, (fa) is the synthetic focal distance of the lens components L1 and L2 and (fb) is the synthetic focal distance of the lens components L3 and L4,βis the image-formation magnification of this optical system, (ra) is the radius of curvature of the bonding surface of the lens component L2, Da is a distance between the bonding surface of the lens component L2 and the aperture diaphragm S andν2p andν2n are the Abbe number of the positive and the negative lenses in the 2nd lens component.
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