Zoom lens system

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 the zoom lens system includes at least one aspherical lens, and
• 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 varies, a distance between the second lens group and the third lens group varies, and a distance between the third lens group and the fourth lens group varies, and the fourth lens group moves along a zoom trajectory having a convex shape facing to an image.

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, and the fourth lens group moves along a zoom trajectory having a convex shape facing an image;
• and the following conditional expression is satisfied:
• 0.003<(Δ4a+Δ4b)/TLt<0.1
• where Δ4a denotes an absolute value of a moving amount of the fourth lens group along the optical axis from the wide-angle end state to a focal length state where the fourth lens group locates the most image side position upon focusing on infinity, Δ4b denotes an absolute value of a moving amount of the fourth lens group along the optical axis from the focal length state where the fourth lens group locates the most image side position to the telephoto end state upon focusing on infinity, and TLt denotes a total lens length of the zoom lens system in the telephoto 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;
• a fourth lens group having positive refractive power, and
• a low-pass filter located between the fourth lens group and an image,
• 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 varies, a distance between the second lens group and the third lens group varies, and a distance between the third lens group and the fourth lens group varies, and the fourth lens group moves along a zoom trajectory having a convex shape facing the low-pass filter.

A zoom lens system consisting of, 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, and the fourth lens group moves along a zoom trajectory having a convex shape facing an image.

A method for changing a focal length of a zoom lens system that includes, 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 the zoom lens system includes at least one aspherical lens, and wherein variation of lens group positions from a wide-angle end state to a telephoto end state comprises steps of:

• varying a distance between the first lens group and the second lens group;
• varying a distance between the second lens group and the third lens group;
• varying a distance between the third lens group and the fourth lens group;
• and
• moving the fourth lens group along a zoom trajectory having a convex shape facing an image.

A method for changing a focal length of a zoom lens system that includes, 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, and wherein variation of lens group positions from a wide-angle end state to a telephoto end state comprises steps of:

• 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;
• varying a distance between the third lens group and the fourth lens group;
• and
• moving the fourth lens group along a zoom trajectory having a convex shape facing an image;
• and
• wherein the following conditional expression is satisfied:
• 0.003<(Δ4a+Δ4b)/TLt<0.1
• where Δ4a denotes an absolute value of a moving amount of the fourth lens group along the optical axis from the wide-angle end state to a focal length state where the fourth lens group locates the most image side position upon focusing on infinity, Δ4b denotes an absolute value of a moving amount of the fourth lens group along the optical axis from the focal length state where the fourth lens group locates the most image side position to the telephoto end state upon focusing on infinity, and TLt denotes a total lens length of the zoom lens system in the telephoto end state.

A method for changing a focal length of a zoom lens system that includes, 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, a fourth lens group having positive refractive power, and a low-pass filter located between the fourth lens group and an image and wherein variation of lens group positions from a wide-angle end state to a telephoto end state comprises steps of:

• varying a distance between the first lens group and the second lens group;
• varying a distance between the second lens group and the third lens group;
• varying a distance between the third lens group and the fourth lens group;
• and
• moving the fourth lens group along a zoom trajectory having a convex shape facing the low-pass filter.