This is NOT a focus adjustment or a range finder. This adjustment allows your scope to adjust the optics, so that the crosshairs will be correctly positioned when shooting at different distances. This is accomplished by adjusting the front lens to remove the parallax, making sure the image focuses on the same plane as the crosshairs . . . . not in front or behind it.

Most scopes have a fixed parallax setting that is exactly correct for shooting at only "one" specific distance.

If your scope has a parallax adjustment it can be perfectly adjusted for shooting at any distance, so that your crosshairs will appear to be rock solid, no matter where your eye is positioned.

The crosshairs should appear stationary, as if they were painted on your target. This is very important when the "exact" position of your eye is not always concentric with your scope.

The slightest variance can make a huge difference.