What is parallax, why do we care, and how do we correct these errors when shooting? Well, hopefully I can clear up those questions in plain English here.
First up, what the heck is parallax? Dictionary.com defines parallax as the apparent displacement of an observed object due to a change in the position of the observer. While this is helpful for understanding how to catch a parallax error when looking down a scope, it doesn’t quite tell us what it is.
A parallax error occurs when the shooter is looking through a scope and the focus is not on the same plane as the reticle. Why does this matter? Simple, if parallax is present and your head or eye position shifts out of line with the center of the scope, your reticle will be off target. Which is definitely not good.
So now that we know what it is, how do we recognize an error? There’s actually a pretty simple method. When looking through the scope at your target, shift your head slightly. Either up and down or left and right. If the reticle changes position on your target, you have a problem. If the reticle stays in the same place on the target, you’re good to go.
So let’s say you have parallax present, how do you fix it? Easy. Most quality rifle scopes, especially those with higher magnification, will have a parallax adjustment. It is usually the knob on the left of the scope that is marked with yardage. To adjust, first rotate the knob to the distance you are shooting at, then shift your head back and forth slightly to check for the reticle moving around on the target. Make fine adjustments if needed until the crosshairs stay put and no parallax error is present.
Here’s a great little 2 minute video that explains this. Pay attention to the scope views in this video that simulate the shooter moving their head around while looking through the scope. Remember that as you do it yourself.