The adjustable objective (AO) does not determine the range for you. In fact, you have to pretty much know the range you are trying to shoot in order to use AO on the scope.
It DOES focus the scope, but not precisely enough to use the readings on the AO for rangefinding. It's purpose is to remove or reduce the phenominon known as parallax. This is very important for precise shooting at extreme ranges (extremely far or close).
Parallax is the apparant movement of the crosshairs on the target when the eye is moved slightly while viewing through the scope. If the scope is not set to be "parallax free" at the distance you are shooting, then placing your eye at slightly different locations behind the scope can cause the crosshairs to appear to be located in slightly different places on the target. This makes shooting small groups difficult, since aiming at the exact same spot each time is paramount for small groups. Once adjusted properly, you can move your eye around to different places while viewing through the scope and the crosshairs will appear to remain steady or "parallax free".
Most non-adjustable scopes are set to be parallax free at about 100 yards. The further (or closer) you get from that distance, the worse the parallax appears.
AO scopes are very useful on .22 rifles since most of the shooting is done at distances less than 100 yards. When using scopes with high magnification, it becomes impossible to even get a focused sight picture at close range without an AO on the scope.
Mikey
