At least one exemplary embodiment is directed to a zoom lens system, which includes a first, a second, a third, and a fourth lens units of positive, negative, positive, and positive refractive powers, respectively, in this order from an object side to an image side. At a telephoto end, the first lens unit is closer to the object side, a distance between the first and second lens units is larger, a distance between the second and third lens units is smaller, and a distance between the third and fourth lens units is larger, as compared to those at a wide-angle end. The first lens unit includes negative and positive lens elements. The second lens unit includes negative and positive lens elements in this order from the object side to the image side.