Parallax is a displacement or difference in an object's apparent position observed along two distinctive sightlines. Parallax is measured by the angle or semi-angle of inclination between the two lines.
Parallax is a factor with scopes, red dot sights, and iron sites.
Parallax occurs when the target and reticle are on different planes within the scope. It is noticeable when you move your cranium or eye around while gazing through the optic, and the reticle seems to shift or whirl around the target.
Parallax is produced by an inconsistency in cheek weld and height of eye positioning when mounting a rifle, which results in the reticule inaccurately reflecting where the gun is aimed. Misalignment can cause the scope to be off-target contingent on the eye's alignment within the optic and lead to poorly placed shots.
It requires the shooter to place both the reticle and target on the same focal plane.
A modest example of parallax error can be found on any iron-sighted gun. Move your head up, down, left, or right, and the front sight will no longer be in the correct position of the rear sight. With iron sights or fixed sights, parallax error is eliminated by focusing on the front sight to keep it centered (Equal light and height) in the rear sight.
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