| 摘要 |
PROBLEM TO BE SOLVED: To provide an image reading lens system which is relatively bright with an F number of about 5.6, has a relatively great depth of field, and provides 110% or more aperture efficiency at a half field angle of about 30°when reading an image reduced to about -0.05 times. SOLUTION: The image reading lens system includes, in order from the object side: a first lens which is a positive meniscus lens the convex surface of which is oriented on the object side; a diaphragm; a second lens which is a negative meniscus lens the concave surface of which is oriented on the object side; and a third lens which is a biconvex positive lens. At least one surface of the first lens is aspherical. The aspherical surface has such a shape that positive power increases toward its peripheral part from a part near its axis, thereby increasing aperture efficiency. COPYRIGHT: (C)2009,JPO&INPIT
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