I know this is probably a really stupid question, but I am new to the scope world. I will keep it simple. I have a Savage .270 that came with a scope. Just recently I picked up a Weaver Classic Extreme 2.5-10x50 scope and it has a knob with yards marked on it. 100-600 maybe, cant remember the high number off the top of my head. I am familiar with windage and e elevation adjustments, but what would this knob be for? Is it really as simple as turning it to the yards I'm shooting at?