You know parrallax was somthing i always struggled with in college. I guess understanding how to calculate distances between the objective and errector lenses or build a scope at a particular parrallax distance. Of course our college crude optics they had made it tough.

But basically when you look at an object at 400 yards say thru your crosshairs, and you move you head around (eye), you want the crosshairs to be locked on your target not dance. Now with a AO set to 500 yards, you will see this movement up close at say 25 yards.