| 摘要 |
PURPOSE:To obtain the small-sized, lightweight zoom telescope which has a wide visual field and various aberrations, such as a spherical aberration, reduced by allowing an afocal zoom lens system which has a four-group lens system to meet specific requirements. CONSTITUTION:Lenses in four groups L1-L4 are provided successively toward an object. Fixed lenses L1 and L4 have positive focal lengths and moving lenses L2 and L3 have negative focal lengths; and the lens L1 is a biconcave lens, the lens L3 is a cemented achromatic biconcave lens, and the lens L4 is a positive single lens. The lens L1 is a positive three-element lens consisting of a cemented achromatic lens which consists of two elements in one group and has convex surface on the object side, or a positive meniscus lens having the convex surface on the object side and a cemented achromatic lens which consists of two elements in one group and has the convex surface on the object side. Specially, the former cemented achromatic lens satisfies conditions shown by inequalities I -IV, where eta2 and nu2, and eta3 and nu3 are refractive indexes and Abbe numbers on its line (d) successively toward the object. |
