| 摘要 |
A front group consists of two concave lenses. A rear group consists of: a positive lens in which the object-side surface has a larger radius of curvature; and a cemented lens configured by a positive lens in which the object-side surface has a larger radius of curvature, and a negative meniscus lens, the cemented lens having being positive as a whole. An objective lens satisfies the following conditional expressions: <?in-line-formulae description="In-line Formulae" end="lead"?>|d<SUB>x</SUB>/f<SUB>F</SUB>|>=3.0 (1)<?in-line-formulae description="In-line Formulae" end="tail"?> <?in-line-formulae description="In-line Formulae" end="lead"?>(f/f<SUB>3</SUB>)xnu<SUB>3</SUB><23 (2)<?in-line-formulae description="In-line Formulae" end="tail"?> <?in-line-formulae description="In-line Formulae" end="lead"?>f<SUP>2</SUP>x(nu<SUB>5</SUB>-nu<SUB>4</SUB>)/{R<SUB>A</SUB>x(Bf+d<SUB>5</SUB>/n<SUB>5</SUB>)}>7 (3)<?in-line-formulae description="In-line Formulae" end="tail"?> where d<SUB>x </SUB>is the distance between the groups, f, f<SUB>F</SUB>, and f<SUB>3 </SUB>are the focal lengths of the objective lens, the front group, and the lens, respectively, nu<SUB>3</SUB>, nu<SUB>4</SUB>, and nu<SUB>5 </SUB>are the Abbe numbers of the lenses, R<SUB>A </SUB>is the radius of curvature of the cementing surface between the lenses, B<SUB>f </SUB>is the back focus, and d<SUB>5 </SUB>and n<SUB>5 </SUB>are the center thickness and refractive index with respect to the d-line of the lens.
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