| 摘要 |
PROBLEM TO BE SOLVED: To provide a high performance imaging optical system that is small and ensures the illuminance of the periphery of an image screen. SOLUTION: The imaging optical system includes, in order from the imaging object side, a first lens group with positive refracting power, a second lens group with negative refracting power, a third lens group with positive refracting power, and a fourth lens group with positive refracting power. The fourth lens group is composed of a negative lens and a positive lens in that order from the imaging object side. Magnification is varied by moving at least the first and third lens groups. The imaging optical system satisfies a relation expressed by 1<f1(fW×fT)<SP>1/2</SP><4, wherein f1 denotes the focal distance of the first lens group, fT denotes the focal distance of the entire imaging optical system at the telephoto end, and fW denotes the focal distance of the entire imaging optical system at the wide-angle end. COPYRIGHT: (C)2007,JPO&INPIT
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