Patent ID: 11940607
Assignee: FUJIFILM CORPORATION
Field: Optics (Instruments)
Classification: CPC G | IPC G

Claim 0:
1. A variable magnification optical system comprising, as lens groups consecutively in order from an object side to an image side:
a first lens group;
a second lens group;
a third lens group; and
a fourth lens group,
wherein all intervals between adjacent lens groups are changed during changing magnification, and
in a case where a d-line back focus in an air-conversion distance of the variable magnification optical system at a telephoto end is denoted by Bfd, a back focus in any one wavelength from a wavelength of 1300 nm to a wavelength of 2325.42 nm in the air-conversion distance of the variable magnification optical system at the telephoto end is denoted by BfIR, a d-line focal length of the variable magnification optical system at the telephoto end is denoted by ft, a focal length of the first lens group is denoted by f1, a focal length of the second lens group is denoted by f2, a focal length of the third lens group is denoted by f3, a refractive index in F line is denoted by nF, a refractive index in C line is denoted by nC, a refractive index in a wavelength of 1970.09 nm is denoted by n1970, and a partial dispersion ratio θ is defined as θ=(nC−n1970)/(nF−nC) for each lens of all lens groups, and an average of θ of all lenses of the third lens group is denoted by θ3 ave, Conditional Expressions (1), (25-4), (30) and (4) are satisfied, which are represented by

−0.0022<(Bfd−BfIR)/ft  (1)

0.3<ft/f1<6  (25-4)

−10<f3/f2<−0.1  (30)

1.4<θ3 ave  (4).