| 摘要 |
PROBLEM TO BE SOLVED: To obtain a zoom lens having large aperture ratio of about 1.8 F-number at a wideangle end which is provided with a large diameter ratio and high variable power ratio of about 13-20 power by regulating the arrangement of a lens elements, the apportionment of aberration correction and the apportionment of achromatism so that a prescribed condition is satisfied. SOLUTION: A 1st group is constituted of a 11th group fixed at the time of a focusing action and a 12th group moved to an object side when the focusing action is executed to the object at a short distance from the object at an infinite distance. The 11th group is constituted of a combined lens obtained by sticking a negative lens L111 and a positive lens L112. The 12th group is constituted of a positive lens L121 and a positive lens L122. Then, when the focal distances of the 1st, the 11th and the 12th groups are respectively defined as f1, f11 and f12, the radius of the curvature of an (i)th lens surface is defined as Ri, the refractive index of the material and the Abbe's number of the lens L111 are respectively defined as N111 andν111 and the refractive index of the material and the Abbe's number of the lens L112 are respectively defined as N112 andν112, the condition of an expression is satisfied. Thus, the fluctuation of the spherical-surface aberration and the chromatic aberration occurring in accordance with a variable power action and the focusing action is reduced.
|
