| 摘要 |
PURPOSE: To shorten the overall length of a zoom lens while imparting a high variable power ration to a rear focus type zoom lens of five-group constitution by setting the refracting power of lens groups and the moving conditions of the second group and the fourth group at variable power and adopting the constitution for moving the fourth group at the time of focusing. CONSTITUTION: This zoom lens consists of the first lens group I having positive refracting power, second lens group II having negative refracting power, third lens group III having positive refracting power, fourth lens group IV having positive refracting power and fifth lens group V having negative refracting power. The second lens group II is moved at the time of varying the power from a wide angle end to a telephoto side and the image plane fluctuation accompanying power variation is corrected by moving the fourth lens group IV. The third lens group III has a positive lens group 3a and a negative lens 3b having a strong concave face on the image side. The condition 0.45<|f3a /f3|<0.75 is satisfied when the focal length of the lens group 3a is defined as f3a and the focal length over the entire part of the lens group as f3 in order to shorten the overall length of the lens by forming the third lens group III as a required sufficient telephoto type.
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