Parallax is the apparent shift of an objects position relative to more distant background objects caused by a change in the observers position. It is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or half-angle of inclination between those two lines. Parallax arises due to a change in viewpoint occurring due to the motion of the observer, of the observed, or both.

Parallax is used in various fields, including astronomy, photography, and optics. In astronomy, parallax is an irreplaceable tool for calculating distances of faraway stars. The first successful measurements of a stellar parallax were made by Friedrich Bessel in 1838, for the star 61 Cygni. In photography, parallax is used to reproduce and display 3D images. In optics, parallax affects sighting devices of ranged weapons such as rifle scopes, binoculars, and microscopes.

In rifle scopes, parallax describes a situation where the focal plane of the object in the scope is offset from the reticle. Parallax compensation changes neither the focus of the reticle nor the focus of the image, it simply moves the planes at which these two objects are in focus so that they share the same plane (are coincident) . By eliminating parallax and having the target image and reticle on the same plane, you no longer have to have a precise line of sight: no matter what angle you are looking through, the scope at the reticle will still accurately indicate the correct point of aim.