Hawke Vantage 3-9x40mm AO Riflescope Parallax Correction (AO) in a 3-9x40mm Scope is not all that common. But the Hawke Vantage 3-9x40 AO with Adjustable Objective lets you dial out parallax aiming error at all distances from 15yds to infinity. Parallax error is when the crosshair changes when you move your head. Dial it out for any exact distance with the Adjustable Objective on this scope. For shooting the tightest groups parallax adjustment is critical. But for hunting situations, you just set it to the general distance you expect to shoot and forget it... Just like any fixed parallax scope. But, when you need parallax error to be dialed out, it sure is nice to have a scope with an adjustable objective... Error free aiming is a must for the best accuracy.