Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight. A simple example is as follows: Extend your arm and point at a distant object looking through one eye only. Now imagine that the object is a deer and your finger is the crosshair of your riflescope. Move you head right then left and observe how the point of aim shifts.
Parallax in a riflescope happens when your eye moves away from dead center of the field of view and the point of impact (aiming point) shifts. This occurs when the reticle is not in the exact same focal plane of the object being viewed.
This is from Wikipedia:
If an optical instrument "” e.g., a telescope, microscope, or theodolite "” is imprecisely focused, its cross-hairs will appear to move with respect to the object focused on if one moves one's head horizontally in front of the eyepiece. This is why it is important, especially when performing measurements, to focus carefully in order to eliminate the parallax, and to check by moving one's head.
Most 3-9 scopes that I am familiar with have parallax set at 100 yards. "Shotgun" type scopes are frequently set to be parralex-free at 50 or 75 yards. Riflescopes of greater magnification 6-18 for example, cover a greater range of magnification (12 in 6-18 vs 6 in a typical 3-9) sotheymust have a focus adjustment in order to get the reticle and focal point in the same plane in order to eliminate parallax.
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PARALLEX ??
