Zoom lens system with vibration reduction

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

• a first lens group having positive refractive power;
• a second lens group having negative refractive power;
• a third lens group having positive refractive power;
• and
• a fourth lens group having positive refractive power,
• wherein when a state of lens group positions varies from a wide-angle end state to a telephoto end state, a distance between the first lens group and the second lens group increases, a distance between the second lens group and the third lens group decreases, and a distance between the third lens group and the fourth lens group varies,
• the third lens group consists of, in order from the object, a front lens group having positive refractive power and a rear lens group having negative refractive power, and
• only the rear lens group is capable of shifting in a direction substantially perpendicular to an optical axis to correct an image blur caused by a camera shake, and
• wherein the following conditional expression is satisfied:
• 1.70<Bfw/fw<3.00
• where fw denotes the focal length of the zoom lens system in the wide-angle end state and Bfw denotes the back focal length in the wide-angle end state.

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

• a first lens group having positive refractive power;
• a second lens group having negative refractive power;
• a third lens group having positive refractive power;
• and
• a fourth lens group having positive refractive power,
• wherein zooming is carried out by varying distances between respective lens groups,
• the third lens group consists of, in order from the object, a front lens group having positive refractive power and a rear lens group having negative refractive power,
• only the rear lens group is capable of shifting in a direction substantially perpendicular to an optical axis to correct variation in focusing position,
• at least one lens surface in the rear lens group is an aspherical surface having a shape that positive refractive power becomes stronger or negative refractive power becomes weaker from the optical axis to the periphery than a spherical surface having a paraxial radius of curvature, and
• the following conditional expression is satisfied:
• 3.5<f1/fw<8.0
• where f1 denotes the focal length of the first lens group and fw denotes the 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 comprises, in order from the object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power;

• the method comprising steps of:
• varying the focal length by increasing 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, and varying a distance between the third lens group and the fourth lens group when the state of the zoom lens system varies from a wide-angle end state to a telephoto end state;
• providing the third lens group that has, in order from the object, a front lens group having positive refractive power and a rear lens group having negative refractive power;
• shifting only the rear lens group in a direction substantially perpendicular to an optical axis to correct an image blur caused by a camera shake;
• and
• satisfying the following conditional expression:
• 3.5<f1/fw<8.0
• where f1 denotes the focal length of the first lens group and fw denotes the 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 comprises, in order from the object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power;

• the method comprising steps of:
• varying the focal length by increasing 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, and varying a distance between the third lens group and the fourth lens group when the state of the zoom lens system varies from a wide-angle end state to a telephoto end state;
• providing the third lens group that has, in order from the object, a front lens group having positive refractive power and a rear lens group having negative refractive power;
• shifting only the rear lens group in a direction substantially perpendicular to an optical axis to correct an image blur caused by a camera shake;
• and
• satisfying the following conditional expression:
• 1.70<Bfw/fw<3.00
• where fw denotes the focal length of the zoom lens system in the wide-angle end state and Bfw denotes the back focal length 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 comprises, in order from the object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power;

• the method comprising steps of:
• varying the focal length by varying distances between respective lens groups when the state of the zoom lens system varies from a wide-angle end state to a telephoto end state;
• providing the third lens group that has, in order from the object, a front lens group having positive refractive power and a rear lens group having negative refractive power;
• shifting only the rear lens group in a direction substantially perpendicular to an optical axis to correct variation in focusing position;
• providing an aspherical surface on at least one lens surface in the rear lens group, wherein the aspherical surface has a shape that positive refractive power becomes stronger or negative refractive power becomes weaker from the optical axis to the periphery than a spherical surface having a paraxial radius of curvature;
• and
• satisfying the following conditional expression:
• 3.5<f1/fw<8.0
• where f1 denotes the focal length of the first lens group and fw denotes the focal length of the zoom lens system in the wide-angle end state.