An optical system includes a front lens unit, a first focusing unit having a negative refractive power, and a second focusing unit having a negative refractive power in this order from an object side to an image side. The optical system is a single focus optical system. When a focus is shifted from an object at infinity to an object at a short distance, the first focusing unit and the second focusing unit move to change a distance between the first focusing unit and the second focusing unit on an optical axis.