| 摘要 |
PURPOSE: To provide a projection zoom lens having a variable power ratio of about two, a half viewing angle >=30 deg. at the wide-angle end and excellent performance. CONSTITUTION: This zoom lens is composed, in order from the screen side S, of a first group I having a positive refractive power, a second group II having a negative refractive power and a third group III having a positive refractive power, the focal distance of the whole system is changed by changing the interval between the first and the second groups on the optical axis and the interval between the second and the third groups on the optical axis, and the focal distance at the wide-angle end is fW, the back focal length at the wide-angle end is bfW, the focal distance of the first group is f1 (>0) and the lateral magnification of the third group at the wide-angle end isβ3 W (<0) satisfy the conditions: (1) bfW/fW>1.6, (2) 5<f1 /fW<10, (3) 0.5<|β3 W|<1.0.
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