Zoom lens system and image pickup apparatus including the same

A zoom lens system comprising:

• a first lens unit having a positive optical power;
• a second lens unit having a negative optical power, the second lens unit being positioned on an image side of the first lens unit;
• a third lens unit having a positive optical power, the third lens unit being positioned on the image side of the second lens unit;
• a fourth lens unit having a positive optical power, the fourth lens unit being positioned on the image side of the third lens unit;
• and
• an F-number determining member that determines a luminous flux of a minimum F-number, the F-number determining member being positioned between a vertex of an object-side lens surface of a lens that is nearest to an object side in the third lens unit and an intersecting point of the object-side lens surface and a peripheral portion of the lens in a direction of an optical axis,
• wherein the second lens unit, the third lens unit, and the fourth lens unit move during zooming.