May 13, 2020
Ever head out to do some shooting and notice that your rifle’s point of impact is slightly different than your last range visit? It’s usually not a big difference, but it’s also not uncommon for the zero to wander anywhere from one-half to one-inch.
I blamed the optic mounting system for this issue for years, as do many shooters I know. I figured the scope moved in the rings a tiny bit or the rings or base shifted some. While all those potential issues are somewhat valid (especially when using poorly made equipment), the far more likely scenario is the presence of parallax inside the scope.
Any riflescope has two functional optical components: The eyepiece and everything else. The “everything else” is the scope forward of the power adjustment ring. As the image enters the scope through the objective lens, it is manipulated, focused and dumped into the first focal plane located right underneath the turrets. The erector assembly takes the image, magnifies it and focuses it into the second focal plane located at the power-adjustment ring. The eyepiece works like a microscope and allows the shooter’s eye see what’s in the second focal plane.
There are two optical components that need to be focused together to totally eliminate parallax. The first is the image coming onto the scope. This is easy to do because all it requires is looking through the scope while turning the side-focus turret until the image is tack-sharp. Once that image is in focus, it is perfectly located in the second focal plane, where the eyepiece is ready for it.
Most of us have learned that one of the first steps in setting up a scope involves focusing the reticle. Point the scope at a light-colored background, look at the reticle for 2 to 3 seconds and then look away. Next, turn the diopter a half-turn and then look at the reticle for another 2 to 3 seconds. This part is just like an eye test. If the reticle gets sharper, keep going in that direction until it is as sharp as possible. If the reticle becomes fuzzier, turn the diopter in the opposite direction.
Never stare at the reticle while turning the diopter. The human eye tries to focus whatever is in front of it, so as the reticle’s focus is changing, so is the eye. Staring at the reticle while adjusting its focus results in a poorly set-up and out-of-focus optic.
The number one cause of parallax in a scope is improper or incomplete reticle focus. When the reticle isn’t in focus, this means the eyepiece is looking either in front of, or behind the second focal plane for the incoming image. The reality is that it needs to be looking precisely at the second focal plane.
An indicator that there is still some parallax error in any riflescope is the difficulty knowing when the reticle is most in focus. The ability to spin the diopter a half-turn and see no change in reticle focus likely means there will be parallax in the scope with the resulting wandering zero. The good news is getting that last little bit of parallax out of the scope and eliminating a wandering zero is not overly difficult. Start by focusing the reticle as described above. Point the scope at a neutral background, look at the reticle for a few seconds, look away, and adjust the diopter. Repeat until the reticle is in focus.
Now it’s time for step two. Put up a target with a clearly defined and small aiming point. A 1-inch orange or bright yellow dot on a piece of cardboard (or Hornady’s peel-and-stick target) work nicely. Use the side-focus knob to get the target sharp. Bag the gun in place so it stays on target without any human support. Without putting your face on the stock, look through the scope and move your head up and down and side to side. If there is no reticle movement relative to the target, there is no parallax in the scope. If there is reticle movement, make a 1⁄8- to 1/4-turn adjustment of the diopter and perform the side-to-side and up-and-down head movement again. Continue to make small adjustments to the diopter until there is no reticle movement, even when you move your head around behind the scope.
It should require no more than two full revolutions of the diopter to eliminate the parallax using this supplementary method. If you find yourself struggling to eliminate that last tiniest bit of reticle movement, know that some scopes will have a small amount of parallax no matter what you do. I teach a handful of precision rifle courses a year, and out of 10 students, one or two will always have some parallax, even with both myself and the student working with the scope. That statistic covers what I’d call “professional-grade” scopes. Any optic made in the Philippines or China will have a much higher incidence of permanent parallax. Go ahead and get mad at me, but them’s the facts.
Performing small focus changes to the diopter while keeping the target image in-focus is the best way to get both target and reticle focused simultaneously. Once both focal planes align, there is no parallax in the system, so variations in head placement will not cause the point of impact to move. Any shooter that takes the time to do the above will likely see slightly smaller groups because he’s successfully eliminated one more variable from the shooting equation.
Of course, this is all just mumbo jumbo without a way to check. The check is simple and requires 20 rounds and a 100-yard range. Before using the above process, put 10 1-inch dots on a piece of paper and put the target up at 100 yards. Get into a good shooting position from the prone or shooting bench and fire one round at the first dot. Stand up and come completely off the rifle, then get back behind the rifle, move to the second dot, and fire one round. Repeat this process until you’ve shot one round at all 10 dots.
Go through the steps outlined above to eliminate all parallax and then conduct the drill a second time. You’ll find the rounds land more often in the same relative location than before. Overlaying all the round impacts will also show a smaller composite
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