Patent ID: 6859331

Claim:
An objective having lens units, each of which is constructed with single lenses, and a numerical aperture of 0.7 or more, the objective comprising, in order from an image side: a first lens unit including a positive meniscus lens with a convex surface facing an 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 a radius of curvature of a surface of the negative lens, adjacent to the positive lens, is smaller than a radius of curvature of an 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, the objective satisfying the following condition: 0 <|R min/ R max|<0.5 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.