Zoom lens system, imaging apparatus and method for varying focal length

An imaging apparatus including a zoom lens system, the zoom lens system comprising, in order from an object side:

• a first lens group having negative refractive power;
• a second lens group having positive refractive power;
• a third lens group having negative refractive power;
• and
• a fourth lens group having positive refractive power;
• upon varying a focal length from a wide-angle end state to a telephoto end state, a distance between the first lens group and the second lens group decreasing, a distance between the second lens group and the third lens group increasing, and a distance between the third lens group and the fourth lens group decreasing;
• the second lens group consisting of two positive lenses or less and one negative lens;
• the fourth lens group consisting of two positive lenses or less and one negative lens;
• the following conditional expressions being satisfied:
• 25≦νd2n≦40
• when 25≦νd2n≦30
• −0.035×νd2n+2.63≦nd2n≦1.79
• when 30<νd2n≦35
• 1.58<nd2n≦1.79
• when 35<νd2n≦40
• 1.58<nd2n≦−0.034×νd2n+2.98
• where νd2n denotes Abbe number of the negative lens in the second lens group at d-line, which has a wavelength λ=587.6 nm, and nd2n denotes a refractive index of the negative lens in the second lens group at d-line;
• and
• the following conditional expressions being satisfied:
• 25≦νd4n≦40
• when 25≦νd4n≦30
• −0.035×νd4n+2.63≦nd4n≦1.79
• when 30<νd4n≦35
• 1.58<nd4n≦1.79
• when 35<νd4n≦40
• 1.58<nd4n−0.034×νd4n+2.98
• where νd4n denotes Abbe number of the negative lens in the fourth lens group at d-line, and nd4n denotes a refractive index of the negative lens in the fourth lens group at d-line.

A method for varying a focal length of a zoom lens system that includes a first lens group, a second lens group, a third lens group, and a fourth lens group, the method comprising steps of:

• providing the first lens group having negative refractive power, the second lens group having positive refractive power and consisting of two positive lenses or less and one negative lens, the third lens group having negative refractive power, and a fourth lens group having positive refractive power and consisting of two positive lenses or less and one negative lens;
• varying a focal length from a wide-angle end state to a telephoto end state by decreasing a distance between the first lens group and the second lens group, increasing a distance between the second lens group and the third lens group, and decreasing a distance between the third lens group and the fourth lens group;
• satisfying the following conditional expressions:
• 25≦νd2n≦40
• when 25≦νd2n≦30
• −0.035×νd2n+2.63≦nd2n≦1.79
• when 30<νd2n≦35
• 1.58<nd2n≦1.79
• when 35<νd2n≦40
• 1.58<nd2n≦0.034×νd2n+2.98
• where νd2n denotes Abbe number of the negative lens in the second lens group at d-line, which has a wavelength λ=587.6 nm, and nd2n denotes a refractive index of the negative lens in the second lens group at d-line;
• and
• the following conditional expressions being satisfied:
• 25≦νd4n≦40
• when 25≦νd4n≦30
• −0.035×νd4n2.63≦nd4n≦1.79
• when 30<νd4n≦35
• 1.58<nd4n≦1.79
• when 35<νd4n≦40
• 1.58<nd4n≦−0.034×νd4n+2.98
• where νd4n denotes Abbe number of the negative lens in the fourth lens group at d-line, and nd4n denotes a refractive index of the negative lens in the fourth lens group at d-line.