| 摘要 |
An objective has lens units, each of which is constructed with single lenses, and a numerical aperture of 0.7 or more, comprising, in order from the image side, a first lens unit including a positive meniscus lens with a convex surface facing the image side, at least two negative lenses, and at least two positive lenses, and having negative power as a whole; a second lens unit including a negative lens and a positive lens so that the radius of curvature of the surface of the negative lens, adjacent to the positive lens, is smaller than that of the opposite surface thereof; a third lens unit including biconvex positive lenses and biconcave negative lenses which have different media, so that two of the biconvex positive lenses are arranged on the object side and the image side, and having positive power as a whole; and a fourth lens unit including a negative meniscus lens and at least one positive meniscus lens, and having positive power as a whole. In this case, the objective satisfies the following condition:where Rmin is the radius of curvature of the surface of the negative lens, adjacent to the positive lens, in the second lens unit and Rmax is the radius of curvature of the opposite surface thereof.
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