Parallax is essentially an optical illusion. Parallax presents itself as the apparent movement of the reticle, in relation to the target, when your eye moves off center of the sight picture (exit pupil) or in more extreme cases it appears as an out of focus image. It indicates that the scope is either out of focus or more specifically the image of the target is not occurring on the same focal plane as the reticle.
Maximum parallax occurs when your eye is at the very edge of the sight picture (exit pupil). Even when parallax is adjusted for a designated distance, there is an inadvertent error at other distances.
Most brands of scopes that do not have a parallax adjustment are pre-set at the factory to be parallax free at or around 100 yards; rim fire and shotgun scopes are set at or around 50 yards. Most scopes of 11x or more have a parallax adjustment because parallax worsens at higher magnifications.
Generally speaking parallax adjustment is not required for hunting situations and is primarily a feature used and desired by target shooters. A 4x hunting scope focused for 150 yards has a maximum error of only 8/10ths of an inch at 500 yards.
At short distances, the parallax effect does not affect accuracy. Using the same 4x scope at 100 yards, the maximum error is less than 2/10ths of an inch. It is also good to remember that, as long you are sighting straight through the middle of the scope, or close to it, parallax will have virtually no effect on accuracy in a hunting situation.
This is another way to think of it that maybe you can relate to. You know when you are sitting in the passenger seat of a vehicle it’s hard to look at the speedometer and tell how fast you are going because your eye….the needle….and the mph number are not all three lined up.
So to you it looks like you’re going 35 when really you are going 55. But the person behind the steering wheel has his eye, the needle and the mph all lined up straight in the same focal plane and gets a true reading. It’s not exactly the same thing but close enough for government work.