| 摘要 |
PROBLEM TO BE SOLVED: To obtain an objective optical system having visual field conversion optical elements of a small size and high performance with a smaller number of lens elements by forming the faces on the image side of third lenses which are relatively high in abaxial ray height as convex shapes in order to correct abaxial aberrations and further, satisfying a specific condition. SOLUTION: This objective optical system comprises, successively from an object side, first lenses r1, r2 of negative lenses, the visual field conversion optical elements r3, r4, second lenses r6, r7 of positive lenses and third lenses r10, r11 of positive lenses. The first lenses r1, r2 are planoconcave shapes and the second lenses r6, r7 and third lenses r10, r11 are concave-convex shapes, from the object side. A diaphragm r5 is arranged between the visual field conversion optical elements r3, r4 and the second lenses r6, r7. The faces on the image side of the third lenses r10, r11 are convex shape. The condition 0.1<|r(3R)/f|<20 is satisfied. In the equation, r(3R) is the radius of curvature of the faces on the image side of the third lenses and (f) is the focal length of the entire system.
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