| 摘要 |
PROBLEM TO BE SOLVED: To provide a variable-magnification image forming optical system where at least chromatic aberrations and distortion aberrations are corrected satisfactorily, and which has high resolution and a wide visual field. SOLUTION: A front group G<SB>F</SB>having positive refractive power, a diaphragm S, and a rear group G<SB>R</SB>having positive refractive power are arranged, in this order starting from the object side. The lens 11, positioned closest to the object side of the front group, has a concave face on the object side. The lens 22, positioned closest to the image side of the rear group, has a concave face on the image side. When varying the magnification of the image forming optical system 10 from the low magnification side to the high magnification side, the front group and the rear group are separately moved to the object side, while reducing the arrangement distance of the diaphragm (aerial gap D1). The maximum image height Y' of the image forming optical system 10, the focal distance fm of the image forming optical system at the lowest magnification, the focal distance f<SB>F</SB>of the front group and the focal distance f<SB>R</SB>of the rear group satisfy the following conditional inequalities: 0.15<Y'/fm<0.25, 1.1<f<SB>F</SB>/fm<1.5 and 1.3<f<SB>R</SB>/fm<1.7. COPYRIGHT: (C)2006,JPO&NCIPI
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