| 摘要 |
The focal length of the lens system is changed by moving first to third lens groups along the optical axis. The negative first lens group contains positive and negative lens elements cemented together, the surface nearest the objective side being concave. The zoom lens system fulfills the condition -1 less than r1/fW less than 0 , 1.4 less than f1G/f1N less than 1.8, where r1 is the radius of curvature of the concave surface of the first lens group, fW is the focal length of the whole zoom lens system when the shorted focal length is set, f1G is the focal length of the negative first lens group, and f1N is the focal length of the negative lens elements of the cemented lens elements.
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