Patent ID: 6741399
Filing Date: 2004-05-25
Classification: G02B

Abstract:
A four-group zoom lens comprising, in order from the object side:a first lens group having positive refractive power; a second lens group having negative refractive power that includes, from the object side, a biconcave or a plano-concave lens with the flat surface of the plano-concave lens or the concave surface of the biconcave lens facing the object side, another biconcave lens, and a positive meniscus lens with its convex surface facing the object side, the adjacent lens surfaces of the positive meniscus lens and said another biconcave lens having the same curvature and being fixed together in contact with one another or separated by a layer of transparent adhesive that fixes the lenses together; a third lens group having positive refractive power and consisting of a single lens with at least one lens surface thereof being aspherical; a fourth lens group having positive refractive power and at least one lens surface that is aspherical; wherein the first lens group and the third lens group are fixed in position; the second lens group is moved along the optical axis when zooming; the fourth lens group is moved along the optical axis for both focusing and zooming; and the following condition is satisfied: 3.1<f4/fw<3.6 wheref4 is the focal length of the fourth lens group, and fw is the focal length of the zoom lens at the wide-angle end.