| CPC G02B 15/144 (2019.08) [G02B 13/14 (2013.01); G02B 15/143105 (2019.08); G02B 15/144105 (2019.08); G02B 15/144113 (2019.08); G02B 15/16 (2013.01)] | 15 Claims |
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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).
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