The adjustable objective (AO) allows adjusting the scope to be free of parallax error at different distances. Parallax is rarely an issue unless you are shooting over 200m (250 yards). When hunting this can be a drawback if you encounter targets that are closer than that. The AO is generally mounted on the Objective Lens Bell at the front of the scope, this means it is hard to reach when shooting. Therefore the AO scope tends to be used at fixed distances such as benchrest competitions.Â