That has to be one of the most common questions concerning scopes. Scope manufacturerís explanations might as well be written in a foreign language because you often find youíre even more confused after reading their definition. Speaking of definitions, here is what Webster has to say parallax is:
par-al-lax n. - An apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight. The apparent displacement of an object caused by a change in the position from which it is viewed. Parallax of the cross wires (of an optical instrument), their apparent displacement when the eye changes its position, caused by them not being exactly in the focus of the object glass. Binocular parallax, the apparent difference in position of an object as seen separately by one eye, and then by the other, the head remaining unmoved.
Iíve found over the years that most people do not know what parallax is and those that do, have a very hard time explaining it to others. Iíve found two examples that are very helpful in understanding parallax.
You know when you are sitting in the passenger seat of a vehicle itís 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 your 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 plane and gets a true reading. This is not exactly the same but it really helps to get your mind thinking in the right direction.
With one eye closed hold your thumb out in front of your open eye and put your thumb on top of a distant object. Now close the eye you are looking through and open the other eye while you hold your thumb steady on target. Your target is now visible and your thumb will have shifted to the left or right. The target did not move nor did your thumb but somehow they are not on top of each other any longer because the observation point changed. This is not exactly the same as parallax in a scope either but again it helps set the stage.