Optical system and imaging device

An optical system consisting of:

• a first lens group and a second lens group in order from an object side, wherein
• during focusing from infinity to a short distance, the first lens group is fixed with respect to an image plane and the second lens group moves along an optical axis,
• the first lens group has negative refractive power, includes at least one negative lens, and includes a positive lens closest to the image side,
• the second lens group consists of a lens subgroup 2A having positive refractive power, an aperture stop, and a lens subgroup 2B having positive refractive power in order from the object side, and
• the lens subgroup 2B including a negative lens and a positive lens in order from the object side, satisfying the following Conditional Expressions (1) and (2):
• -
• 0.050
• <
• θ
• ⁢
• ⁢
• IRp
• -
• θ
• ⁢
• ⁢
• IRn
• <
• 0.050
• (
• 1
• )
• 1.50
• <
• F
• ⁢
• ⁢
• 2
• /
• F
• <
• 2.60
• (
• 2
• )
• where
• θIRp is an average of (nF−nd)/(n1700nm−nd) of all positive lenses included in the second lens group,
• θIRn is an average of (nF−nd)/(n1700nm−nd) of all negative lenses included in the second lens group,
• nF is a refractive index at a wavelength of the F line,
• nd is a refractive index at a wavelength of the d line,
• n1700nm is a refractive index at a wavelength of 1700 nm,
• F2 is a focal length of the second lens group at the wavelength of the d line, and
• F is a focal length of the optical system at the wavelength of the d line at infinity focus.