| 摘要 |
PURPOSE:To obtain a lens which is capable of chromatic aberration compensation required for a mode hop countermeasure and has a joint surface formed into a spherical surface, by constituting the lens of the first double convex lens whose object-side face is formed into an aspherical surface and the second negative lens stuck to the image-side face of the first lens and satisfying specific conditions. CONSTITUTION:This objective lens consists of the first double convex lens whose object-side face is aspherical and the second negative lens stuck to the image-side face of the first lens, and conditions of formulas are satisfied. In this case, a positive lens having a large Abbe's number and a lens having a small Abbe's number are joined to compensate the chromatic aberration, it is necessary to properly determine the radius of curvature of the joint surface. The first condition specifies this radius of curvature; and if the radius of curvature of the joint surface is larger than the upper limit, it is difficult to increase the numerical aperture though compensation of the chromatic aberration is advantaged. If the radius of curvature is shorter than the lower limit, the curvature of the object-side face of the double convex lens is too high to compensate the chromatic aberration, and the sine condition is degraded.
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