Rifle shooter Number One is a pretty good shot and decides to add a scope to his rifle. He buys a good quality red dot scope, some pretty good factory ammunition and he heads out to the range to sight in the rifle at 100 yards. To improve his accuracy, he rests the rifle on a shooting bench so that there will be virtually no movement of the gun during the firing sequence. He then spends some time firing a round, checking the impact on the target and then adjusting the scope until the bullets hit the target where the red dot is superimposed. Finally, he has a small group in the middle of the target. Success!
At this point, his buddy shows up and shooter number one invites him to shoot the rifle. Shooter number Two is also a pretty good shot and he shoots a tight group, but all his shots are off to one side of the target. Shooter number One then shoots another group, which, like his first group is in the center of the target. How can two shooters with good quality rifle, ammunition and scope put tight groups on different places on the target? It could be that parallax in the scope is the answer.
Parallax causes the apparent displacement of an observed object due to the lenses and the distance to the target. In other words, the light from the target that causes an image of the target to register in the shooter’s eye is bent. The light is not straight. This means that that an object (the target) being observed through the scope is not exactly where you see it. The degree of displacement, through the same optic can vary from one shooter to the next. In this example, both shooters saw through the scope the same image of the dot in the center of the target, but in reality, they were aiming the gun at two slightly different places on the target. Distance to the target increases the problem. At 200 yards, shooter number two might see his group on the target even farther off to one side than it was at 100 yards.