| 摘要 |
PURPOSE: To provided a zoom lens which is about 3 in zoom ratio, about 2.0 to 2.8 in F value at a wide angle end, is as wide as about 60 deg. in viewing angle and is smaller in size by specifying the constitution of three groups. CONSTITUTION: The first group G1 has a positive refracting power and the second group G2 has a negative refraction power and the entire part is movable along the optical axis for the purpose of variable magnification and focusing. The absolute value of its magnification is always smaller than 1. The third group G3 has a positive refracting power, is movable along the optical axis at the time of variable magnification and moves in such a manner that the relation between the position on the optical axis and the focal length of the entire system is monotonous. The zoom lens satisfies the conditions expressed by 0.8<2γN/2γINF<1.0, 0.25<D12 W/fW<0.5, 0.4<LW/LT<1.0. In the equations,2γN is the variable power ratio at the time of focusing at the short distance object point of the second group G2;2γINF is the variable power ratio at the time of focusing at the infinite distance object point of the second group G2, D12 W is the air spacing of the first group G1, the second groups G2 on the optical axis at the time of focusing at the infinite object point at the wide angle end, fW is the focal length of the entire system at the time of focusing at the infinite object point at the wide angle end; LW is the moving quantity at the wide angle end of the second group G2 when focused from a certain object point to a certain object point, LT is the moving quantity at the telephoto end of the second group G2 of this time.
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