Optical system, imaging device having same, and method for manufacturing optical system

An optical system comprising, disposed in sequence along an optical axis starting on an object side:

• a first lens group having a positive refractive power, which remains fixed along an optical axis relative to the image surface upon focusing;
• a second lens group having a positive refractive power, which moves along the optical axis upon focusing;
• and
• a third lens group having a positive refractive power, which remains fixed along the optical axis relative to the image surface upon focusing, wherein:
• the first lens group comprises a partial lens group that comprises at least two positive lenses and has, as a whole, a positive refractive power, and a negative lens;
• the second lens group comprises a positive lens, a negative lens and a cemented lens having a positive refractive power, disposed in sequence starting on the object side;
• the third lens group comprises a cemented lens;
• and
• a following conditional expression is satisfied:
• −1.00<(r2nb+r2na)/(r2nb−r2na)<0.00
• where:
• r2na:
• a radius of curvature of an object side lens surface of the negative lens in the second lens group;
• and
• r2nb:
• a radius of curvature of an image side lens surface of the negative lens in the second lens group.

A method for manufacturing an optical system that comprises a first lens group having a positive refractive power, which remains fixed relative to an image surface upon focusing, and a second lens group having a positive refractive power, which moves along an optical axis upon focusing, disposed in sequence along the optical axis starting on an object side, the method comprising:

• achieving that the first lens group comprises:
• a first partial lens group that comprises at least two positive lenses and has a positive refractive power;
• and
• a negative lens;
• achieving that the second lens group comprises:
• at least one meniscus lens with a convex surface thereof facing the object side;
• at least one negative lens, disposed toward an image surface side relative to the meniscus lens;
• a second partial lens group that is disposed next to and on the image surface side of a negative lens having a highest negative refractive power, among the at least one negative lens, comprises at least one positive lens and has a positive refractive power;
• and
• a cemented lens that is disposed on the image surface side of the second partial lens group, is formed by bonding together a plurality of lenses and has a positive refractive power;
• achieving that a lens surface located closest to the image surface side in the second partial lens group and a lens surface located closest to the object side in the cemented lens have a convex shape facing the image surface side and sit opposite each other via an air gap;
• and
• achieving that a following conditional expression is satisfied
• −1.00<(rbc2−rbc1)/(rbc2+rbc1)<0.00
• where:
• rbc1:
• a radius of curvature of the lens surface in the second partial lens group located closest to the image surface side;
• and
• rbc2:
• a radius of curvature of the lens surface of the cemented lens located closest to the object side.

A method for manufacturing an optical system that comprises a first lens group having a positive refractive power, which remains fixed along an optical axis relative to an image surface upon focusing, a second lens group having a positive refractive power, which moves along the optical axis upon focusing, and a third lens group having a positive refractive power, which remains fixed along the optical axis relative to the image surface upon focusing, disposed in sequence along the optical axis starting on an object side, the method comprising:

• achieving that the first lens group comprises a partial lens group which comprises at least two positive lenses and has, as a whole, a positive refractive power, and a negative lens;
• achieving that the second lens group comprises a positive lens, a negative lens and a cemented lens having a positive refractive power, disposed in sequence starting on the object side;
• achieving that the third lens group comprises a cemented lens;
• and
• achieving that a following conditional expression is satisfied:
• −1.00<(r2nb+r2na)/(r2nb−r2na)<0.00
• where:
• r2na:
• a radius of curvature of an object side lens surface of the negative lens in the second lens group;
• and
• r2nb:
• a radius of curvature of an image side lens surface of the negative lens in the second lens group.