Parallax And Optical Centre

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Parallax is one of those optical terms that can be confusing until a person sits down and does their own personal test to demonstrate its existence. Simply put, it refers to the difference in apparent position a rifle scope’s reticle can occupy on a target, based on the location of the shooter’s eye. It’s best observed by placing a scoped rifle on a set of sandbags and looking at a target through the scope without touching the rifle in any way. As the eye is moved back and forth, the reticle will appear to change position. This happens because the reticle is not in the same focal plane as the target. A manufacturer can only set one point at which there is no parallax – on a hunting scope it’s often around 100 yards. Any other distance will show some parallax. But some scopes have adjustments to correct for parallax at any distance beyond a certain minimum. This adjustment can usually be found on the objective lens or as a dial mounted on the left side of the scope’s main tube. However, these adjustments can potentially make a scope larger, heavier, more expensive and offer more opportunity for leaking and damage.

Traditional wisdom says that because of the target size involved in big game hunting, parallax adjustment isn’t a needed feature. However, for target shooting or varmint hunting, where the ranges can be long and the targets small, it’s a desirable extra. I’ve generally subscribed to those ideas and the scopes on my rifles reflect it. However, I thought I should actually test those concepts to see just how valid they really are.

To do so, I took a couple of rifles with representative scopes to the shooting range. The first was a Tikka T3 with a simple Leupold 3-9X scope and fixed parallax. The second was another Tikka, this one an older 695 model, equipped with a Leupold 12X target scope with parallax adjustment. The plan was to shoot both rifles at varying distances. The first group from each rifle would be shot with my eye as perfectly centered in the scope’s field of view as I could manage. The second group would be shot with my eye deliberately misplaced. Would there be a difference in accuracy and, if so, how much? The table below shows group sizes that resulted from my testing.

As you can see, one of the significant results was that the groups shot with the fixed parallax scope doubled in average size when the eye was off centre. This indicates how critically important it is to keep the eye centered when trying to make an accurate shot with a fixed parallax scope. It also demonstrates that shooting such a scope accurately can be done; after all, a 0.89 minute of angle (MOA) average out to 300 yards is quite respectable for a sporter-weight rifle with a 9X scope. All that’s necessary is to keep the eye carefully centered in the scope’s field of view.

But look at the results for the adjustable parallax scope. Even though the parallax was adjusted for each range, the group size still grew significantly when the eye was misplaced behind the scope. At first this surprised me, but once I started to think about it a little more, I realized it did make sense, especially at the longer ranges. After all, every scope has an optical centre that is engineered to be the sweet spot for that optical instrument. In that centre zone, there is minimum distortion and objects appear at their maximum clarity. Move the eye out of that zone, as I did deliberately, and even though the parallax may be gone there will still apparently be an effect on group size.

That brings us to what I think is the most valuable lesson to be learned from this little exercise, which is simply that for maximum accuracy, the shooter’s eye must be in the centre of the scope’s view, even if the rifle scope has adjustable parallax. Move the eye outside of that sweet spot and bad things will happen, whether you shoot a parallax adjustable scope or not.

 

Top photo: With the eye properly centred, when looking through a rifle scope, the shooter will see a full circle with clearly defined edges.

Bottom photo: When the eye is out of the scope’s optical centre, a crescent shaped shadow creeps into view.

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