One of the common shooters failure are do not understand right what is these mystical rifle scope parallax? Mentioned on every rifle scope manual, this important setting on rifle scope adjustment is ofter misunderstood as focus setting, which is definitely wrong as focus totally different thing. In a few words: parallax is an credible elocution or difference in the credible area of an object viewed along two dissimilar lines of sight, and this value is measured only by the semi-angle or on other cases angle of inclination between those two viewing lines.

Or in simple explanation, rifle scope parallax setting is positioning your optical scope aiming reticle for shooting at different shooting distances, after adjustment you focus your eye sight, reticle and aiming object on same plane as scope lenses. So simple.