| 摘要 |
PROBLEM TO BE SOLVED: To provide a large aperture standard lens system that is a negative lens precedence type, suitable for image processing used to highly accurately inspect an object in a finite distance, and has a large back focus and an F number of 1.4 or so. SOLUTION: The large diameter standard lens system includes, in order from the object side, a positive front group lens, a diaphragm, and a positive rear group lens. The front group lens includes, in order from the object side, a negative lens group 1a and a positive lens group 1b. The 1a lens group is composed of three lenses which are, in order from the object side, a first negative lens, a second positive lens, and a third negative lens. The large aperture standard lens system satisfies following conditional expressions (1) to (3): (1) 0.5<f/¾f<SB>1a</SB>¾<0.75, (2)1.0<¾f<SB>1a</SB>¾/f<SB>1b</SB><1.4 and (3) 0.3<<SB>d1a-3</SB>/f<1.0, wherein f represents the focal distance of the entire system, f<SB>1a</SB>is the focal distance (f<SB>1a</SB><0) of the 1a lens group, f<SB>1b</SB>represents the focal distance (f<SB>1b</SB>>0) of the 1b lens group, and d<SB>1a-3</SB>represents the thickness of the third negative lens of the 1a lens group. COPYRIGHT: (C)2006,JPO&NCIPI
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