A lens system is disposed on the image side of an image pickup optical system and has a negative refractive power. The lens system includes, in order from an object side to the image side, a first lens unit having a positive refractive power, a second lens unit, and a third lens unit. The first lens unit consists of a single lens or a cemented lens. The second lens unit is configured to move in a direction of an optical axis to change a spherical aberration. A lateral magnification of the lens system and a lateral magnification of the second lens unit in a case where an axial ray is incident on the image pickup optical system are appropriately set.