Mike Morton runs you through what “parallax” error refers to and how to check it and correct it…
Parallax is the effect whereby the position or direction of an object appears to differ when looked at from different positions. Parallax error therefore occurs when the object is not viewed from the correct position. In shooting terms, parallax error is an unwelcome phenomenon that’s usually associated with telescopic sights, when the target image is not focused on the reticle of the scope.
It’s easy enough to check for parallax error. Look through your scope and line up the crosshairs over your target, then move your eye slightly off-centre. If the crosshairs remain centred over the target, there is no parallax error; but if you move your eye and find the crosshairs are no longer over the target, you have a problem. If you were to make some shots and take no account of parallax error, your pellets would most likely not all land in exactly the same place.
Luckily, most scopes can be corrected for parallax error, with the help of built-in objective or side-focus parallax adjustment controls. First, adjust the parallax control until the target is perfectly in focus, then check for any parallax error by moving your eye slightly off-centre. The target image should now be perfectly focused on the reticle, and the crosshair should remain over the target.
Parallax error changes with distance, so you’ll need to readjust your controls every time you alter the distance over which you’re shooting. Parallax error can also be minimised by keeping your eye central to the ocular lens for every shot you take, which is why maintaining a good, repeatable head position on your stock is so important.