An optical system comprising a first lens unit disposed closest to an object side, a first focusing unit disposed on an image side of the first lens unit, and a second focusing unit disposed on the image side of the first focusing unit, wherein the first focusing unit and the second focusing unit move in different loci during focusing, wherein in a first arrangement state, the first focusing unit and the second focusing unit are so arranged to generate a first spherical aberration while maintaining an in-focus state at a predetermined object distance, and in a second arrangement state, the first focusing unit and the second focusing unit are so arranged to generate a second spherical aberration different from the first spherical aberration while maintaining the in-focus state at the predetermined object distance, characterized in that the first arrangement state is able to be changed to the second arrangement state by moving the first focusing unit and the second focusing unit only, and focusing is performed by moving the first focusing unit and the second focusing unit, wherein a total length of the optical system, which is a distance between a most object side surface of the first lens unit and an image plane, is the same in the first arrangement state and the second arrangement state, the first arrangement state and the second arrangement state are arrangement states of the optical system that is in focus on an infinite object, the following conditional expression is satisfied: 0.9 < β i1 × β i 2 / β j1 × β j 2 < 1.1, where βi1 is the lateral magnification of the first focusing unit in the first arrangement state, βi2 is the lateral magnification of the second focusing unit in the first arrangement state, βj1 is the lateral magnification of the first focusing unit in the second arrangement state, and βj2 is the lateral magnification of the second focusing unit in the second arrangement state. wherein the first focusing unit and the second focusing unit move in different loci during focusing, wherein in a first arrangement state, the first focusing unit and the second focusing unit are so arranged to generate a first spherical aberration while maintaining an in-focus state at a predetermined object distance, and in a second arrangement state, the first focusing unit and the second focusing unit are so arranged to generate a second spherical aberration different from the first spherical aberration while maintaining the in-focus state at the predetermined object distance, characterized in that the first arrangement state is able to be changed to the second arrangement state by moving the first focusing unit and the second focusing unit only, and focusing is performed by moving the first focusing unit and the second focusing unit, wherein a total length of the optical system, which is a distance between a most object side surface of the first lens unit and an image plane, is the same in the first arrangement state and the second arrangement state, the first arrangement state and the second arrangement state are arrangement states of the optical system that is in focus on an infinite object, the following conditional expression is satisfied: 0.9 < β i1 × β i 2 / β j1 × β j 2 < 1.1, where βi1 is the lateral magnification of the first focusing unit in the first arrangement state, βi2 is the lateral magnification of the second focusing unit in the first arrangement state, βj1 is the lateral magnification of the first focusing unit in the second arrangement state, and βj2 is the lateral magnification of the second focusing unit in the second arrangement state.