| 摘要 |
PROBLEM TO BE SOLVED: To obtain a compact and bright image-formation lens securing a long back focus and a wide field angle and realizing the reduction of cost in the case of mass production by making the lens a retrofocus type lens constituted of two concave and convex lenses and satisfying a specified conditional expression. SOLUTION: The elementary constitution of this lens is the retrofocus type and is constituted of two lenses L1 and L2 , and a diaphragm (i) is disposed between the 2nd lens L2 and a low-pass filter 1, then a luminous flux made incident along an optical axis X from an object side forms an image at the image-formation position P of a solid-state image pickup element 2. The 1st lens L1 has negative refractive power and is a biconcave lens whose one surface, at least, is aspherical, and the 2nd lens L2 is a biconvex lens whose both surfaces have nearly equal curvature. The ranges of the values of the refractive index (n) and the Abbe numberνof the 2nd lens L2 consisting of the biconvex lens are specified, and they satisfy the conditional expressions: 1.55<=n and 35<=ν, thereby, the lens is advantageous in terms of the correction of coma aberration and astigmatism.
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