| 摘要 |
PURPOSE:To obtain a document photographing lens reduced at its size and cost by satisfying a lens system obtained by sequentially arraying the 1st-3rd lenses from the object side to the image side with prescribed conditions. CONSTITUTION:The 1st, 2nd and 3rd lenses are sequentially arrayed from the object side to the image side. The 1st lens 10 consists of a convex lens having convex surfaces on both the sides, the 2nd lens 12 is a concave lens having concave surfaces on both the sides and the 3rd lens 14 is a convex lens having convex surfaces on both the sides. When it is defined that the composite focal distance of the whole system is (f), the radius of curvature of the surface of the i-th lens counted from the object side is ri(i=1-6), the distance between the surfaces of the i-th lens is di(i=1-5), the refractive index of the i-th lens is ni(i=1-3), and the Abbe number of the i-th lens is nui(i=1-3), these values Ri, di, ni, nui, and f are set up so as to be satisfied with the conditions shown by equalities.
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