A single focal length lens includes an aperture stop and first through fourth lens units having, respectively, negative refractive power, positive refractive power, negative refractive power, and positive refractive power. Positions of the first, second, and fourth lens units are fixed. When focusing from an object at infinity to an object at a short distance, the third lens unit moves toward the image side. Conditional expressions (1), (2), and (3) are satisfied:−5<；f1/f<；−0.5 (1),−10<；f3/f<；−1 (2), and0.5<；f12/f<；1 (3),where f1 denotes a focal length of the first lens unit, f denotes a focal length of an overall single focal length lens system when focusing to an object at infinity, f3 denotes a focal length of the third lens unit, and f12 denotes a combined focal length of the first and second lens units.
