Parallax & Accuracy Testing
After spending hours sorting brass and bullets, measuring every aspect to .001 and finally creating “that” load to beat all others, you go to the range and all hopes and dreams vanish within the first three shots. Why?? Last week these worked great.. What happened?
I’ve seen this time and again. This issue is insidious at best. We trust our gear and the instructions they come with. But, sometime there are details we overlook or just take for granted. This article will cover one that needs to be understood by all “precision driven” reloaders and marksman.
How does Scope Parallax impact group size? Let us review.
What IS Scope Parallax anyway?
Parallax is essentially an optical illusion. Parallax presents itself as the apparent movement of the reticle, in relation to the target, when your eye moves off center of the sight picture (exit pupil) or in more extreme cases it appears as an out of focus image. It indicates that the scope is either out of focus or more specifically the image of the target is not occurring on the same focal plane as the reticle. Maximum parallax occurs when your eye is at the very edge of the sight picture (exit pupil). Even when parallax is adjusted for a designated distance, there is an inadvertent error at other distances. Most brands of scopes that do not have a parallax adjustment are pre-set at the factory to be parallax free at or around 100 yards; rim fire and shotgun scopes are set at or around 50 yards. Most scopes of 11x or more have a parallax adjustment because parallax worsens at higher magnifications. Generally speaking parallax adjustment is not required for hunting situations and is primarily a feature used and desired by target shooters. A 4x hunting scope focused for 150 yards has a maximum error of only 8/10ths of an inch at 500 yards. At short distances, the parallax effect does not affect accuracy. Using the same 4x scope at 100 yards, the maximum error is less than 2/10ths of an inch. It is also good to remember that, as long you are sighting straight through the middle of the scope, or close to it, parallax will have virtually no effect on accuracy in a hunting situation.
Example.. For parallax you really only need to think of the scope as having two planes, one is where the image is formed and the second is where the reticle is focused. A target 1000 yards away will come to focus at a very specific distance behind the objective lens. A target at 100 yards will come to focus at a different location in the scope’s tube further from the objective than the 1000 yard image. The parallax adjustment on a scope simply moves the reticle’s plane to be in the same location as the image plane. We are talking about very minute distances, like .1mm, which does not sound like much but it is compounded by magnification. Each power of the scope will multiply any error in parallax. So let’s say you have adjusted the parallax the best you can and you missed aligning the image plane and reticle plane by .1mm, the .1mm misalignment would change as you change the power. For simplicity sake let’s pretend we have a 1-20x scope (wouldn’t that be nice!). We have it initially set on 1x and we adjust the parallax as close as we can but we missed aligning the image plane and reticle plane by .1mm, now we turn the magnification up to 20x and we have compounded the error x20. “Provided by Rifle Scope School”
So now let’s add a few points for the precision reloader. The sentence “Most scopes of 11x or more have a parallax adjustment because parallax worsens at higher magnifications” should be in BOLD, since most precision marksman purchase the highest magnification possible to allow the greatest level of precision (aim small miss small). What they didn’t say is, “when sitting at your bench, target at 100 yards adjust the parallax dial on the left side of your scope by rolling the dial until the 100 mark on the cap is alongside the witness line on the scope body”… you may not have eliminated the parallax error yet!
Remember, the 100 yard mark on the parallax cap is ONLY a reference mark. To truly eliminate parallax, steady your rifle to the greatest amount possible then do this:
1) while looking through your scope, line up your horizontal reticle with a thin horizontal line on your paper target.
2) Back your eye away from the scope so the image is reduced by half. This will force your eye to be more aligned with the center of the scope lens.
3) Move your head up and down slightly like your nodding yes to someone
4) IF the your reticle moves off the line as you move your head, adjust the parallax dial in either direction (slightly) until nothing moves (regardless of what the number says on the parallax dial. Some scopes deal with this better than others. Now your parallax is adjusted properly.
If you have NOT been performing this step properly each time you start your shooting session you’ve not optimized the rifle and scope to perform at their best. Your loads could be outstanding but if you place your cheek (and eye) at a slightly different place on the stock each time, parallax error will make you think you’re aiming at the same spot, but the scope and your eyes are lying to you.
For a true indication of your loads accuracy, you need to ensure you’ve eliminated parallax error before shooting. Remember to do this every time you change your shooting distance!
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