| 摘要 |
PROBLEM TO BE SOLVED: To provide a small-diameter objective optical system suitable for in vivo observation with a high numerical aperture by reducing the outer diameter and effectively correcting various aberrations. SOLUTION: The invention provides a small-diameter objective optical system comprising, in order from an object plane a first lens group with positive refractive power, including at least one plano-convex lens whose convex surface faces an image plane; a second lens group with positive refractive power, including at least one concave lens; and a third lens group including a cemented lens whose cemented surface has negative refractive power. The focal length of the third lens group is longer than the focal length of the first lens group. COPYRIGHT: (C)2009,JPO&INPIT
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