US Patent 9678305 Inner focus lens

An inner focus lens includes sequentially from an object side, a first lens group having a positive refractive power; a second lens group having a negative refractive power; and a third lens group having a negative refractive power. The second lens group is moved along the optical axis, whereby focusing from a focus state for an object at infinity to a focus state for a minimum object distance is performed. The inner focus lens satisfies a conditional expression (1) −29.0≦f3/f≦−5.4, where f3 is a focal length of the third lens group at the focus state for an object at infinity and f is a focal length of the optical system overall at the focus state for an object at infinity.