| 摘要 |
PURPOSE:To obtain a lens system which has excellent image forming performance over a wide range and can photograph at a close distance, by satisfying the prescribed conditions for a lens system consisting of the 1st converging group, the 2nd converging group and the 3rd diverging group successively from the side of a subject. CONSTITUTION:The 1st converging lens group, the 2nd converging lens group and the 3rd diverging lens group are provided successively from the side of a subject. A diaphragm is provided between the 1st and 3rd lens groups; the 1st and 2nd lens groups can move in a focusing mode; and at least a negative lens is included in these groups. Thus inequalities (1)-(6) are satisfied, where (dy/dx)I is the differentiation obtained between the air gap (y) between the 1st and 2nd groups and the air gap (x) between the 2nd and 3rd groups when the focusing is performed at a close distance from the infinite point, (dy/dx)II is the differentiation equal to that mentioned above and in a region from the infinite point to the position of 1/10 magnification ratio, nu3 is the Abbe's number of the negative lens of the 1st group, and f, f1 and f2 indicate the focal distances of the whole lens system, the 1st group and the 2nd group, respectively. As a result, a lens system which has excellent image forming performance over a wide range and can photograph at a close distance is obtained. |
