| 摘要 |
PROBLEM TO BE SOLVED: To actualize the zoom lens for projection which has superior properties as a projection zoom lens for a three-plate type liquid crystal projector. SOLUTION: This zoom lens is constituted by arranging 1st to 5th groups in order from the magnification side to the reduction side and providing a diaphragm between the 3rd and 4th groups. The 1st group G1 has negative refracting power and the 2nd and 3rd groups G2 and G3 have each positive refracting power; and the 4th group G4 has negative refracting power and the 5th group G5 have positive refracting power. For variable magnification from the wide-angle end to the telephoto end, the 1st group G1 and 5th group G5 are fixed, the 3rd group G3 moves so that the distance from the 1st group to the 3rd group decreases monotonously, and the 4th group G4 moves so that the distance from the 4th group to the 5th group decreases monotonously; and the diaphragm S moves together with the 3rd group and conditions of 0.8<|f1|/f and 0.8<f5 /f are satisfied, where (f) is the focal distance of the whole system at the wide-angle end, (f1 ) is the focal distance of the 1st group, and (f5 ) is the focal distance of the 5th group.
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