Optical system, method for focusing, and imaging apparatus equipped therewith

An optical system comprising, in order from an object:

• a first lens group having positive refractive power;
• a second lens group having positive refractive power;
• and
• a third lens group having positive refractive power;
• the second lens group being movable along an optical axis for varying focusing, and
• the first lens group satisfying the following conditional expression:
• 2.50<f1/f
• where f1 denotes a focal length of the first lens group, and f denotes a focal length of the optical system.

An optical system comprising, in order from an object:

• a first lens group having positive refractive power;
• a second lens group having positive refractive power;
• and
• a third lens group having positive refractive power;
• the second lens group being movable along an optical axis for varying focusing, and
• the first lens group satisfying the following conditional expression:
• 2.2<f1/f<2.8
• where f1 denotes a focal length of the first lens group, and f denotes a focal length of the optical system.

An optical system having only two lens groups, the two lens groups being, in order from an object:

• a first lens group having positive refractive power;
• and
• a second lens group having positive refractive power;
• the second lens group being movable along an optical axis for varying focusing, and
• at least two negative lenses in the second lens group satisfying the following conditional expressions:
• νdA<50
• 0.00913×νdA+ndA<2.16043
• ndA<1.66000
• where νdA denotes an Abbe number of each of the negative lenses at d-line in which a wavelength λ=587.6 nm, and ndA denotes a refractive index of each of the negative lenses at d-line in which the wavelength λ=587.6 nm.

A method for manufacturing an optical system that includes, in order from an object, a first lens group having positive refractive power and a second lens group having positive refractive power, the method comprising steps of:

• disposing each lens of the first lens group and the second lens group so as to satisfy the following conditional expression:
• 2.50<f1/f
• where f1 denotes a focal length of the first lens group and f denotes a focal length of the optical system;
• and
• disposing the second lens group so as to be movable along the optical axis upon varying focusing, and
• wherein the second lens group weighs as the first lens group or less.

A method for manufacturing an optical system that has only two lens groups, the two lens groups being, in order from an object, a first lens group having positive refractive power and a second lens group having positive refractive power, the method comprising steps of:

• disposing at least two negative lenses in the second lens group satisfying the following conditional expressions:
• νdA<50
• 0.00913×νdA+ndA<2.16043
• ndA<1.66000
• where νdA denotes an Abbe number of each of the negative lenses at d-line in which a wavelength λ=587.6nm, and ndA denotes a refractive index of each of the negative lenses at d-line in which the wavelength λ=587.6nm;
• and
• disposing the second lens group so as to be movable along an optical axis upon varying focusing.

A method for manufacturing an optical system that includes, in order from an object, a first lens group having positive refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power, the method comprising steps of:

• disposing each lens of the first lens group and the second lens group so as to satisfy the following conditional expression:
• 2.50<f1/f
• where f1 denotes a focal length of the first lens group and f denotes a focal length of the optical system;
• and
• disposing the second lens group so as to be movable along the optical axis upon varying focusing.