It ain’t “side focus,” y’all! What’s that parallax dial for on the side of your scope? What does it do and how do you use it? Find out in the video above.

TL:DR version:

• Parallax is an optical illusion that, in the case of a riflescope, causes your reticle to wander based on the position of your eye behind the scope.
• If your parallax isn’t adjusted properly and your eye isn’t perfectly centered, your reticle will appear to be on target…even though it isn’t.
• The scope’s parallax adjustment dial moves the focal plane of the reticle. If your target is at 350 yards, your reticle needs to be on a 350-yard focal plane.
• Adjust your parallax dial and, with the rifle on a solid rest so it won’t move, nod your head slightly behind the scope. As your eye leaves the center of the exit pupil, does the reticle wander off target or does it stay steady? Tinker with that parallax adjustment dial until the reticle doesn’t wander even if your eye is off-center. This is the point where your reticle’s focal plane is precisely at your target distance.
• If your scope doesn’t have a parallax adjustment dial (typically lower magnification scopes won’t and, in those cases, they’re usually set at 100 yards…sometimes 150), or even if it does, a best practice for ensuring your eye is centered in the exit pupil is to adjust your eye relief — the distance between the scope and your eye — such that there’s a thin black border around your scope picture. Like an eclipse. If that black border is perfectly even all the way around the image circumference, then your eye is centered. If your eye is centered, incorrect parallax adjustment won’t matter.

– Jeremy S., Shooting News Weekly