Parallax is one of the most misunderstood terms in optics, but the idea underneath it is simple: it is an apparent shift of the reticle against the target when your eye moves off the center of the scope. Left uncorrected at distance, it introduces aiming error even when your hold and your trigger press are perfect.
What is actually happening inside the scope
A riflescope forms an image of the target on an internal plane, and the reticle sits on a plane of its own. When those two planes coincide exactly, the reticle and the target are locked together: move your head and they move as one. When the target image focuses slightly in front of or behind the reticle, the two are no longer tied, and shifting your eye makes the reticle appear to float across the target.
Hold a finger up at arm's length, line it up with a distant doorway, then move your head side to side. The finger slides across the doorway because the two are at different distances. That is parallax, and a scope that is not focused for the target distance reproduces it in miniature.
How much error does it actually cause?
The error is bounded by how far your eye can move within the exit pupil, and it scales with distance and with how far the parallax setting is off. In practice, a scope set for 100 yards and used at 500 yards can produce an inch or two of apparent shift at 100 yards' worth of scale — which translates into several inches on target at the longer range if your head position varies between shots.
That last clause is the important one. If your cheek weld is identical on every shot, parallax error is consistent and largely disappears into your zero. It is the inconsistency that costs you, which is why parallax error shows up as unexplained vertical and horizontal scatter rather than as a clean point-of-impact shift. Shooters who chase this with scope adjustments often make it worse.
Why it matters more at high magnification
At low power the depth of field is generous and the exit pupil is large, so the target stays effectively in focus across a wide band of distances and small head movements barely matter. At 20x or 25x the depth of field collapses and the exit pupil shrinks, so both the focus error and the sensitivity to head position increase at the same time. This is why precision long-range scopes almost always have a parallax control and a 3-9x hunting scope usually does not.
First: set the eyepiece, and set it once
Almost everyone gets this backwards. The eyepiece, or diopter, focuses the reticle to your eye. The parallax control focuses the target. If the reticle is not sharp to begin with, your eye will keep trying to accommodate and the parallax setting will never feel right.
To set it: point the scope at a plain, bright, featureless background such as a blank wall or open sky. Glance into the scope briefly — a second or two — and note whether the reticle is crisp. Look away, adjust the eyepiece a small amount, and glance again. Short glances matter, because your eye will focus a slightly blurred reticle if you stare at it, and you will dial in the wrong setting. Once it is sharp, lock the ring and leave it alone. It is set for your eyes, not for the rifle or the distance.
Then adjust parallax
Scopes control parallax with an adjustable objective — a ring on the front bell — or, far more commonly on modern designs, a side-focus dial near the turrets. Side focus is easier to reach from behind the rifle and is the better arrangement for prone shooting.
With the rifle steady and the reticle already sharp, turn the control until the target is in focus. Then do the real test: move your head slightly up and down, and left and right, behind the scope. If the reticle stays planted on the same point of the target, parallax is removed. If it drifts, keep adjusting in small increments and re-check.
Trust the head-movement test over the numbers printed on the dial. Those markings are a factory approximation and are frequently off by a meaningful margin, particularly at the extremes of the range and in temperatures far from where they were calibrated. Many precision shooters mark their own dial with tape once they have confirmed true settings at known distances.
A useful shortcut
At practical ranges, sharpest focus and zero parallax occur at nearly the same setting, so focusing by eye gets you very close. The exception is high magnification at long distance, where the two separate slightly. At 25x on a target past 600 yards, finish with the head-movement test rather than relying on focus alone.
Fixed-parallax optics and red dots
Most hunting scopes are factory-set at around 100 yards, and rimfire scopes commonly at 50 yards. Within their intended range that is entirely adequate; a 3-9x40 set for 100 yards has negligible parallax error anywhere from about 50 to 250 yards.
Red dot sights are often described as parallax-free, which is true in the sense that the collimated dot stays on target regardless of where your eye sits in the window. It is not perfectly true in practice — there is usually small residual error at close range — but it is small enough to ignore on a sight used inside 100 yards, and it is the reason a red dot tolerates a sloppy head position so well.
When you actually need an adjustable one
Buy adjustable parallax if you shoot small targets at varying long distances, shoot above about 12x, or shoot rimfire benchrest where targets sit at 50 yards and the factory setting is wrong for you. Skip it on a general-purpose hunting rifle, where it is one more thing to get wrong in a hurry.
To decode the rest of the spec sheet, read how to read riflescope numbers and FFP vs SFP.