Zoom lens system

A zoom lens system comprising, in order from an object:

• 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;
• when a state of lens group positions varies from a wide-angle end state to a telephoto end state, the second lens group, the third lens group and the fourth lens group being moved to the object such that a distance between the first lens group and the second lens group decreases, a distance between the second lens group and the third lens group increases, and a distance between the third lens group and the fourth lens group decreases,
• the first lens group being composed of at least one positive lens and at least one negative lens,
• the second lens group being composed of at least one positive lens and at least one negative lens,
• the third lens group being composed of a single lens,
• the fourth lens group being composed of at least one positive lens and at least one negative lens, and
• the following conditional expression being satisfied:
• 1.70<Bfw/fw
• where Bfw denotes a back focal length of the zoom lens system in the wide-angle end state, and fw denotes a focal length of the zoom lens system in the wide-angle end state.

A method for forming an image of an object and varying a focal length of a zoom lens system that includes, in order from the object, 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, comprising steps of:

• providing the zoom lens system in which the first lens group is composed of at least one positive lens and at least one negative lens, the second lens group is composed of at least one positive lens and at least one negative lens, the third lens group is composed of a single lens, and the fourth lens group is composed of at least one positive lens and at least one negative lens,
• varying the focal length of the zoom lens system from a wide-angle end state to a telephoto end state by moving the second lens group, the third lens group and the fourth lens group to the object such that a distance between the first lens group and the second lens group decreases, a distance between the second lens group and the third lens group increases, and a distance between the third lens group and the fourth lens group decreases, and
• satisfying the following conditional expression:
• −3.00<f3/f4<−0.60
• where f3 denotes a focal length of the third lens group, and f4 denotes a focal length of the fourth lens group.

A method for forming an image of an object and varying a focal length of a zoom lens system that includes, in order from the object, 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, comprising steps of:

• providing the zoom lens system in which the first lens group is composed of at least one positive lens and at least one negative lens, the second lens group is composed of at least one positive lens and at least one negative lens, the third lens group is composed of a single lens, and the fourth lens group is composed of at least one positive lens and at least one negative lens,
• varying the focal length of the zoom lens system from a wide-angle end state to a telephoto end state by moving the second lens group, the third lens group and the fourth lens group to the object such that a distance between the first lens group and the second lens group decreases, a distance between the second lens group and the third lens group increases, and a distance between the third lens group and the fourth lens group decreases, and
• satisfying the following conditional expression:
• 1.70<Bfw/fw
• where Bfw denotes a back focal length of the zoom lens system in the wide-angle end state, and fw denotes a focal length of the zoom lens system in the wide-angle end state.