PURPOSE:To obtain 100% of vignetting factor and to correct aberration satisfactorily, by arranging a diaphragm between a second group and a third group, and specifying the focal distance of a first group, the synthetic focal distance of the first and second groups, and the focal distance of a whole system. CONSTITUTION:The first group faced with an object is formed as a convex meniscus lens 10 whose convex plane faces with an object side, and the second group as a double concave lens 12 having the large plane of radius of curvature at the object side, and the third group provided at an image side as a double convex lens 14, and the diaphragm 13 is arranged between the second group and the third group. Also, assuming the focal distance of the first group as f1, the synthetic focal distance of the first and second groups as f1,2, and the focal distance of the whole system as (f), a device is constituted so as to satisfy conditions 1.50(f/f1)1.80 and -0.90(f/f1,2)-0.40. In such a way, it is possible to obtain a lens in which 100% of vignetting factor can be kept to the most peripheral part by the appropriate distribution of the refractive index and a various kinds of aberration can be corrected effectively without using lanthanum glass with high refractive index, though it is the lens of triplet type constituted of three groups/three sheets.
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