An optical system includes an aperture stop, and an optical member that is removably insertable in an optical path defined by an object side and an image side of the optical system, the optical member is disposed on the image side of the aperture stop. In a first focus range between infinity and a first finite distance, the optical member is removed from the optical path. In a second focus range between a second finite distance shorter than the first finite distance and a third finite distance shorter than the second finite distance, the optical member is inserted in the optical path. The thickness of the optical member on the optical axis and the focal length of the optical member are suitably determined.