Parallax in optics refers to the apparent movement of the reticle against a target caused by the reticle and the image of the target not being on the same focal plane. This can cause accuracy problems for shooters if there is not a consistent cheek weld from shot to shot. Parallax occurs when the target and reticle are on different planes within the scope.

To correct for parallax, most modern rifle scopes have a parallax correction system that involves a dial on the side of the main tube, often referred to as a “side focus”. These systems allow a shooter to adjust the image of the target backward or forward to match the focal plane of the reticle. External parallax adjustments, called adjustable objectives or side focuses, are generally on scopes of more than 10 power or on scopes that are used at close distances.

Before attempting to adjust for parallax, it is important to ensure that the reticle is in focus. This can be done by looking through the scope at a plain, ideally bright background that’s at least five yards away and adjusting the reticle focus ring on the ocular lens until the reticle is sharp and clear to the eye.

In summary, parallax in optics refers to the apparent movement of the reticle against a target caused by the reticle and the image of the target not being on the same focal plane. To correct for parallax, most modern rifle scopes have a parallax correction system that involves a dial on the side of the main tube, often referred to as a “side focus”. Before attempting to adjust for parallax, it is important to ensure that the reticle is in focus.