| 摘要 |
PROBLEM TO BE SOLVED: To obtain an objective lens which is small in size, is wide in angle of view and is well corrected of astigmatisms by providing a first group and a third group with aspherical surfaces, thereby correcting distortion aberrations. SOLUTION: This objective lens consists of the first group having negative power for a wider angle and correction of a Petzval sum, the second group having positive power and the third group having positive power and including the aspherical surface. The plane on the extreme image side of the third group is a plane and satisfies the conditions 0.05<|A2sp/A3As|<20, A2sp<0 and A3As>0. In such a case, A2sp is the number obtained by dividing the cubic coefficient of the astigmatism among the aberration coefficients of Seidel by the spherical surface of the second group by the F-number of the objective lens and A3As is the value obtained by dividing the cubic coefficient of the astigmatism by the aspherical surface of the third group by the F-number of the objective lens. A diaphragm is placed between the first group and the second group and is placed in the front side focal position of the second group and the third group in order to make principal rays incident in parallel on an image guide.
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