Parallax is the effect whereby the position or direction of an object appears to differ when looked at from different positions. Mike Morton explains how to check your scope for parallax error…
Parallax error isn’t just something that affects shooters, because it’s present in everyday life too. The encyclopedia definition of parallax is a ‘displacement or difference in the apparent position of an object when viewed along two different lines of sight, and is measured by the angle of inclination between those two lines’.
That sounds really complicated, so let’s take the example of the driver of a car who’s looking at the needle on the speedometer. He can easily see the
correct speed at which the car is travelling. However, a front-seat passenger looking at the speedo would get a very different reading due to the angle at which they are viewing the needle.
In terms of a telescopic sight, parallax error occurs when the target and reticle are not in the same focal plane. If you look through a scope like this and your eyeline does not exactly match the centreline of the scope, you’ll get a similar effect to the passenger looking at the driver’s speedo. Luckily, most modern scopes are equipped with a side-focusing turret or adjustment ring located on the objective bell, so you can adjust the focal plane of both these elements and eliminate parallax.
Once you’ve adjusted for parallax there’s a simple test you can do to determine whether any error is still present. Lay the crosshairs over a target and move your shooting eye very slightly off the centreline of the scope. If the scope has been correctly parallaxed, the crosshair will still be placed over exactly the same point on the target.
An incorrectly parallaxed scope is one way to spoil your accuracy, and can lead to immense frustration if you think you’ve taken an otherwise good shot, but don’t realise you’ve failed to eliminate parallax error before squeezing the trigger.