At least one exemplary embodiment is directed to a zoom lens system which includes a first lens unit having a positive optical power, a second lens unit having a negative optical power and positioned on the image side of the first lens unit, a third lens unit having a positive optical power and positioned on the image side of the second lens unit, a fourth lens unit having a positive optical power and positioned on the image side of the third lens unit, and an F-number determining member. The F-number determining member can be 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.