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 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:
• θct≥0.800 (1)
• vd≤55 (2)
• nd_pave<1.75 (3)
• vd_pave>50 (4)
• where
• θct is a partial dispersion ratio from a C line to a t line of the negative lens in the part B group,
• the partial dispersion ratio θct from the C line to the t line is defined by a following expression:
• θct=(nC−nt)/(nF−nC)
• nC is a refractive index on the C line,
• nt is a refractive index on the t line,
• nF is a refractive index on an F line,
• vd is an Abbe number on a d line of the negative lens in the part B group,
• nd_pave is an average value of refractive indices on the d line of all positive lenses disposed in the lens subgroup B, and
• vd_pave is an average value of Abbe numbers on the d line of all positive lenses disposed in the lens subgroup B.