| 摘要 |
PROBLEM TO BE SOLVED: To improve optical performance over the whole object distance and over the entire variable power range, and to realize a large aperture and high variable power by specifying the lens constitution of respective lens groups in the constitution of a four-group type one. SOLUTION: This lens is provided with four lens groups, that is, the 1st group L1 to the 4th group L4 in order from the object side. The variable power from the wide angle end to the telephoto end is performed by moving the 2nd group L2 to an image surface side, and the variation of the image surface associated with the variable power is corrected by moving the 4th group L4 to the object side while keeping projecting loci 4a and 4b provided, and focusing is performed by moving the 4th group L4. The 4th group L4 is provided with a 1st lens being positive, a 2nd lens being negative and a 3rd lens being positive, and has at least one aspherical surface. When it is assumed that the focal distance of the 3rd group L3 is f3, the f-number and the focal distance of an entire system at the wide angle end are fNW and fW and the focal distance of the entire system at the telephoto end is fT and they are set as shown by an expression fM=(fW.fT)<1/2> ; they satisfy the condition of 3.44<f3×fNW/fM <15.38.
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