Parallax is an optical issue that comes into play with any telescopic device, including gun scopes. For shooters looking for precision at middling ranges, parallax is a serious issue. While it is not a standard feature, many scopes come with parallax compensators.
What is Parallax?
Parallax is the visual difference of position and/or orientation of an object when it is perceived from two separate lines of sight.
Parallax in Rifle Scopes
Parallax in rifle scopes manifests as the reticle or crosshairs, appearing to be on different points on the target due to slight changes in the position of the eye, relative to the scope's eyepiece.
Parallax is naturally a problem for some kinds of shooting, as these slight changes can put a bullet off course. However, for most shooting purposes, parallax is not an issue, and most scopes do not have parallax compensators for this reason.
The odd thing about parallax and shooting is that the effects are often more noticeable and profound in short-to-medium ranges than for long-range targets. Rifle scope parallax is usually most observable in the 100- to 300-meter range.
Compensating for Parallax
Some rifle scopes compensate for parallax by using a mechanism that puts both the target image and the reticle on the same optical plane. There are two ways of doing this, called "AO" and "Side Focus."
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