Abstract: A four-group zoom lens system that achieves satisfactory brightness and zoom ratio for practical applications and yet is compact and can be manufactured at low cost. The lens system includes in order from the object to the image side, a positive first lens group, a negative second lens group, a negative third lens group, and a positive fourth lens group composed of a positive front lens unit (4.sub.f) and a positive rear lens unit (4.sub.r), the first to third lens groups being capable of moving independently of one another along the optical axis. The front lens unit (4.sub.f) includes in order from the object to the image side, a positive first lens element (4.sub.1) having a convex surface on the image side, a positive second lens element (4.sub.2), a positive third lens element (4.sub.1) having a convex surface on the object side and a negative fourth lens element (4.sub.4) having a concave surface on the object side. The rear lens unit (4.sub.