Optical system and imaging device

An optical system comprising:

• a focus lens group having positive refractive power and configured to move along an optical axis when focusing from an infinite-distance object to a short-distance object, wherein
• the focus lens group integrally moves when focusing, and includes a lens subgroup A, an aperture stop, and a lens subgroup B in order from an object side,
• the lens subgroup A includes a negative lens, a positive lens, and a positive lens in order from an image side,
• the lens subgroup B includes a negative lens and a positive lens in order from the object side, and
• the optical system satisfies following conditional expressions:
• −0.045<θIRp−θIRn<0.045 (1)
• 0.50<Fb/F<2.50 (2)
• where
• θIRp is an average value of (nF−nd)/(n1700 nm−nd) of all positive lenses disposed in the lens subgroup B,
• θIRn is an average value of (nF−nd)/(n1700 nm−nd) of all negative lenses disposed in the lens subgroup B,
• nF is a refractive index on an F line,
• nd is a refractive index on a d line,
• n1700 nm is a refractive index at a wavelength of 1700 nm,
• Fb is a focal length on the d line of the lens subgroup B, and
• F is a focal length on the d line of the optical system.