Zoom lens system and image pickup apparatus equipped with the same

A zoom lens system comprising:

• a first lens unit having positive refractive power;
• a second lens unit having negative refractive power;
• an aperture stop;
• a third lens unit having positive refractive power;
• and
• a fourth lens unit having positive refractive power, wherein the first lens unit, the second lens unit, the aperture stop, the third lens unit, and the fourth lens units are arranged in that order from an object side towards an image side, and wherein the zoom lens system is configured to perform zooming by moving the first through fourth lens units,
• wherein the first lens unit is moved along a locus convex towards the image side when the zoom lens system performs zooming from a wide-angle end to a telephoto end,
• wherein the aperture stop is movable independently of the first through fourth lens units, and
• wherein the following conditions are satisfied:
• −0.6<f2/f3<−0.4, and
• −0.5<MS/M3<0.9
• where MS denotes a moving distance of the aperture stop in an optical-axis direction, M3 denotes a moving distance of the third lens unit in the optical-axis direction, f2 denotes a focal length of the second lens unit, and f3 denotes a focal length of the third lens unit, each moving distance being given a positive sign if the movement is directed towards the image side or a negative sign if the movement is directed towards the object side.