| 摘要 |
PURPOSE:To decrease movable groups, to realize lightweight and transparency, to simplify the structure and to reduce the power consumption by composing the lens system of 1st and 3rd groups which are always fixed, a 2nd group which is movable at the time of power variation, and a 4th group which is movable at the time of focusing and the power variation, and setting specific conditional inequalities. CONSTITUTION:The lens system consists of the 1st and 3rd groups which have positive focal length and is always fixed, the 2nd group which has negative focal length and is variable at the time of the power variation, and the 4th group which corrects a shift in focus position caused at the time of the power variation, and meets requirements shown by the inequalities I-III. The inequality I specifies the composite focal length fAT of the 1st-3rd groups at the longest focal length fT to form a focal constitution between the 3rd and 4th groups, the inequality II increases the focal length f4 of the 4th group to increase the back focus of the whole system, and the inequality III provides a sufficient space for focusing. The number of the movable groups is small, so the lightweight constitution and transparency are realized, the mechanism is simplified, and the low cost and small power consumption are realized.
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