A lot of words have been spoken or written about parallax, but it is still a misunderstood subject in many minds. It can be a difficult concept for people to understand unless explained in simple terms. Parallax is usually irrelevant in lower powered riflescopes and can usually be ignored in scopes such as fixed 4x and 6x power. As we move into variables and higher powered scopes, parallax becomes problematic and must be dealt with.
Firstly, what is parallax? It can be described fairly simply by imagining two vertical sheets of glass. The first, placed at say, fifty metres, has a target drawn on it. The second, placed at say, five metres, has a reticle drawn on it. A simple crosshair will suffice. The observer stands in front of the second pane of glass at a distance of say, five metres, looking through this pane of glass at the distant pane of glass with the target drawn on it.
Superimposing the crosshairs on the target, move your head up and done and left and right. The crosshairs will appear to move on the target. Even a small amount will move the crosshair on the target significantly. The further apart the two panes of glass are, the more significant the movement of the crosshairs will be. As you move the panes of glass closer together, so the difference in the position of the crosshairs on the target will reduce.
When you place the panes of glass against each other, no matter how much you move your head up and down or left and right, the crosshairs will stay superimposed on the target. This means you have moved the target and the crosshairs into the SAME FOCAL PLANE.
This is the essence of parallax adjustment. By adjusting the parallax adjustment, you have reduced the sighting error which can be induced by holding your face on the stock in a slightly different position each time you pull the trigger.