There are provided an optical system and an imaging device including the same. The optical system includes: in order from an object side to an image side, a first lens group having refractive power; and a second lens group having positive refractive power. A variable distance that varies during focusing between the first lens group and the second lens group is closer to an object than a variable distance that varies during focusing between other lenses. The first lens group includes a positive lens and at least one or more negative lenses in order from the image side. The second lens group includes 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. The lens subgroup 2B includes a negative lens and a positive lens in order from the object side and satisfies a predetermined Conditional Expression.

A lens system with a fixed focal length includes five lens groups. The five lens groups include a first lens group, a second lens group configured as a focusing front group, a third lens group having a negative refractive power, a fourth lens group configured as a focusing back group, and a fifth lens group. The fifth lens group has a positive refractive power that is arranged in a stationary manner relative to an imaging plane in a lens barrel and both the focusing front group and the focusing back group are movable jointly relative to one another and to the lens groups arranged in a stationary manner in order to focus the fixed focus length lens system on objects at different object distances.

A lens system with a fixed focal length includes five lens groups. The five lens groups include a first lens group, a second lens group configured as a focusing front group, a third lens group having a negative refractive power, a fourth lens group configured as a focusing back group, and a fifth lens group. The fifth lens group has a positive refractive power that is arranged in a stationary manner relative to an imaging plane in a lens barrel and both the focusing front group and the focusing back group are movable jointly relative to one another and to the lens groups arranged in a stationary manner in order to focus the fixed focus length lens system on objects at different object distances.

The optical system disclosed in the embodiment of the invention includes first to fourth lens groups arranged along an optical axis from an object side toward a sensor side and including each including at least one lens, wherein the first lens group has a refractive power opposite to that of the fourth lens group, the second lens group has a refractive power opposite to that of the third lens group, the first and fourth lens groups are fixed, the second and third lens groups are moved in a direction of the optical axis, each of the first and fourth lens groups includes at least one lens of a non-circular shape, the non-circular lens has a non-circular ratio (CH/CA) greater than 0.7, the non-circular ratio is a ratio of a minimum clear aperture (CH: clear height) and maximum clear aperture (CA) of a lens surface with a large clear aperture among an object-side surface and a sensor-side surface of the non-circular lens, and the non-circular ratio of at least one lens included in the first lens group may be greater than the non-circular ratio of at least one lens included in the fourth lens group.

The optical system disclosed in the embodiment of the invention includes first to fourth lens groups arranged along an optical axis from an object side toward a sensor side and including each including at least one lens, wherein the first lens group has a refractive power opposite to that of the fourth lens group, the second lens group has a refractive power opposite to that of the third lens group, the first and fourth lens groups are fixed, the second and third lens groups are moved in a direction of the optical axis, each of the first and fourth lens groups includes at least one lens of a non-circular shape, the non-circular lens has a non-circular ratio (CH/CA) greater than 0.7, the non-circular ratio is a ratio of a minimum clear aperture (CH: clear height) and maximum clear aperture (CA) of a lens surface with a large clear aperture among an object-side surface and a sensor-side surface of the non-circular lens, and the non-circular ratio of at least one lens included in the first lens group may be greater than the non-circular ratio of at least one lens included in the fourth lens group.