What Is Parallax and How Does It Affect Accuracy
Woody's answers are based on published federal and state law. Not legal advice. Consult a licensed attorney for your specific situation.
Parallax is the apparent shift of your reticle relative to the target when your eye moves behind the scope. It causes point of impact to shift away from where your crosshairs indicate, reducing accuracy.
How This Affects Your Sight Picture
Parallax becomes critical for precision shooting beyond 100 yards, where even small reticle movements translate to significant point of impact shifts. Most rifle scopes have parallax set at the factory for 100-150 yards, but adjustable parallax allows you to eliminate this error at your chosen distance. Understanding and correcting parallax is essential for consistent long-range accuracy.
The Detail
Parallax occurs when the target image and reticle are not on the same focal plane inside your scope. When these two elements exist at different optical distances, moving your eye position behind the eyepiece causes the reticle to appear to move across the target. This isn't just a visual annoyance, it directly affects where your bullet impacts relative to your point of aim.
The amount of parallax error increases with distance and magnification. At 100 yards with a scope set to infinite parallax, you might see minimal shift. Push that same setup to 600 yards and even small head movements can make your reticle appear to jump several inches on target. Higher magnification amplifies this effect because you're essentially magnifying the error along with everything else in your sight picture.
Fixed parallax scopes typically come set for 100 yards or sometimes 150 yards, which works fine for hunting rifles used at moderate distances. Adjustable parallax scopes let you dial out this error at any distance, usually through a side focus knob or an adjustable objective lens. When properly set, your reticle should remain stable on target regardless of minor eye movement behind the scope.
Testing for parallax is straightforward. Set up on a target at your intended shooting distance, get a solid rest, and move your head slightly while keeping the rifle perfectly still. If the reticle appears to shift position on the target, you have parallax error. Adjust your parallax control until the reticle stays put when you move your eye position.
Build Impact
For precision rifle builds intended for long-range work, adjustable parallax becomes non-negotiable rather than a luxury feature. If you're building a hunting rifle for shots inside 300 yards, fixed parallax scopes can save money without sacrificing practical accuracy.