Abstract: A single-focus optical system includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power. The first lens unit includes at least a first sub-unit, a second sub-unit, and a third sub-unit. The first sub-unit includes a negative lens component, the second sub-unit includes two meniscus lens components, and the third sub-unit includes a plurality of positive lens components. The third lens unit includes a front side sub-unit and a rear side sub-unit, and the front side sub-unit includes only a positive lens component. At the time of focusing, the first lens unit is fixed, the second lens unit is movable, and the rear side sub-unit is fixed.