JPS6432217A - Zoom lens device - Google Patents

Abstract

PURPOSE:To make a F number bright with a small number of lens elements, to make a back focus long, and to realize high performance, small size, and lightweight constitution by allowing a lens system consisting of seven elements in two groups in order from an object side to meet prescribed requirements. CONSTITUTION:A lens system consists of a 1st group 1 having negative refracting power and a 2nd group 2 having negative refracting power in order from the object side and varies in power by varying the gap between the groups 1 and 2; and the group 1 consists of a 1st negative meniscus lens L1 which is convex to the object side, a 2nd negative lens L2, and a 3rd positive meniscus lens L3 which is convex to the object side, and the 2nd group 2 consists of a 1st negative meniscus lens L4, a 2nd negative lens L5, a 6th negative lens L6 which has a surface of large curvature on the object side, and a 7th positive lens L7. When the focal length of the whole system is fW, this system has a back focus of >=1.3fW and inequalities I-IV hold. Here, fL, fI, f1, fII, and f7 are the focal length values of the whole system, 1st group, L1, 2nd group, and L7 at telephoto ends, r1 is the radius of curvature of the object-side surface of L1, and n1 and n2 are the refractive indexes of L1 and L2.