| 摘要 |
PROBLEM TO BE SOLVED: To improve the flatness of an image plane and to obtain a good contrast at fluorescent observation by setting the radius of curvature of a third lens group and fourth lens group and the focal lengths of the entire system of an objective lens, the fourth lens group and fifth lens group so as to satisfy specific conditions, by which a Petzval sum is well corrected. SOLUTION: The radius of curvature of the face on the extreme image side of the third lens group G3 is defined as RG3B and the radius of curvature of the face on the extreme image side of the fourth lens group G4 as RG4F. When the focal length of the entire system of the objective lens is defined as (f), the focal length of the fourth lens group G4 as fG4 and the focal length of the fifth lens group G5 as fG5, the equations; 0.4<RG3B/f<3.5, 0.4<|RG4F|/f<3.5, 3.2<fG5/|fG4|<8 are satisfied. As a result, even if the optical material to be used is restricted to the optical material for fluorescence, the compatibility of the well correction of the Petzval sum and various other aberrations is achieved.
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