Parallax is one of the most misunderstood scope terms, but the idea is simple: it is an optical effect where the reticle appears to move against the target when your eye shifts off the center of the scope. Left uncorrected at range, it introduces aiming error even with a perfect hold.
Why it happens
Parallax occurs when the target image and the reticle are not focused on exactly the same plane inside the scope. At close range and low magnification it is negligible. At high magnification and long distance it becomes significant — which is why precision long-range scopes almost always have a parallax adjustment.
How to adjust it
Scopes control parallax with an adjustable objective (a ring on the front bell) or, more commonly, a side-focus dial. To set it: focus the reticle to your eye first using the eyepiece, then adjust the parallax control until the target is sharp and the reticle stops moving when you shift your head slightly. Many dials are marked with distances as a starting point, but trust the "no reticle movement" test over the numbers.
What about fixed-parallax optics?
Many hunting scopes and nearly all red dots have a fixed parallax setting (often around 100 yards for scopes), which is fine for their intended ranges. You only need an adjustable parallax when you shoot small targets at varying long distances. To decode the rest of the spec sheet, read how to read riflescope numbers and FFP vs SFP.
