All You Need to Know About Parallax

Introduction to Parallax

If you deal with optical devices used for measuring or observing purposes, you are likely to run into the topic of parallax. Parallax is the optical phenomenon of two objects on differing planes appearing to move at different rates to the observer. Today we are going to dive into how this can effect your shooting experience; with all you need to know about parallax.

this Primary Arms GLx 3-18 shows off some of the features we discuss today

What is parallax?

If you look at two objects one close and the other far, it is difficult to focus on both at the same time. The parallax adjustment in your riflescope is designed to bring the target image and reticle image together in the scope. We take it for granted that we should be able to see both the target and the reticle clearly through our scopes. When in fact, it is through the hard work of optical engineers who’ve designed them to correct for this characteristic.

**Here you** **can see an illustration of how parallax setting can cause problems**. not having the reticle and target projection aligned can allow your reticle-to-target positioning to vary

Adjustable parallax riflescopes use internal optical correction to adjust the presentation of the reticle to match that of the target. Parallax is the difference in position when the viewing point is moved. Much like distant mountains appear motionless while clouds, trees and other parts of the landscape move when viewed from a moving vehicle.

The same thing can happen within your riflescope, albeit to a lesser degree. But we know it doesn’t take much to induce change in our point of impact. Even your own heartbeat can often be seen as movement through the scope.

Looking Through Your Scope

The eye box is the area where your eye can see through the scope, if you move too far out of the eye box you lose sight picture. And if you move too much within the eye box with uncorrected parallax, you might miss

You can see the effect of parallax sometimes looking through a stationary rifle scope. Without moving the rifle or scope, if you move your eye ever so slightly within the eye-box of the scope. You might see the reticle shift similarly on the target. With nothing physically moving but the eye of the observer, the optical anomaly can be seen.

Apex Optics Rival Desert Tech SRS M2 — Side focus knobs like those found on this **Apex Rival** are used to both focus the image and set parallax

How Does Parallax Effect Your Shooting?

You can imagine now how even the slightest shift in reticle position can effect your bullet’s impact once the trigger is pulled. If your eye is misaligned slightly, any parallax present in the image could cause you to aim incorrectly. The optical misalignment might make you think you’re aiming center of the target when you might be holding a measurable amount off.

This image (right) shows an exaggerated amount of optical misalignment. when parallax is present in your optic this example shows how a good shot can suddenly become a bad one.

We’ve all seen when you move your eye too far left/right or up/down a shadow appears on that side of the image. If you move just a little, before the shadows appear, you might see the reticle wandering on the target. This is because you haven’t adjusted your parallax properly, and you could be in for a miss.

**A closer look at the image from above, you can see how the point of aim has been compromised**. **or at least appears to be**

If you’ve done any shooting at longer distances, you know how often the difference between holding center and even the thickness of the reticle off center can be the difference between a hit and a miss. Removing parallax is important to ensuring you have the correct image while aiming.

**Corrected parallax allows both target and reticle to be seen on the same plane. With both images aligned together there is far less room for error**

Fixed or Adjustable Parallax

Not always but typically any good riflescope designed for long-range shooting will have adjustable parallax. Whereas short range optics like red dots and low power variable optics (LPVO’s) generally have fixed parallax. Cheap scopes typically also come with non-adjustable parallax. It might be fixed at one-hundred yards, or infinity. You should understand this when buying your next scope. If your purpose is shooting up close on a range, 100-yard fixed parallax is fine. Whereas if you plan on shooting precision shots at long-range, fixed parallax at 100-yards would be less ideal.

primary arms PLx Compact 1-8, eotech VUDU 1-8, vector optics Continental 1-10, lpvo, low power variable optic, scope, — These LPVO’s have fixed parallax, which is fine for shorter range shooting. L to R: **Primary Arms PLxC 1-8**, **Eotech Vudu 1-8**, and the **Vector Optics Continental 1-10**. You can read about all of them in our optics reviews

Focus or Parallax?

Most riflescopes with adjustable parallax also use the same control for image focus. Let me clarify; I’m not speaking of the diopter at the rear of the scope used to focus the ocular image. I’m speaking of what is usually a side focus turret or an adjustable objective. Either of these two control options usually does the same thing, it focuses the target image and sets parallax.

winchester model 100 leupold rifle scope — this old Leupold VXII scope has an adjustable objective to compensate for parallax

Perhaps you’ve experienced a binocular where both barrels needed focus adjustment for each of your eyes. Bino’s typically have a diopter on one side, the idea being you use the center focus knob to focus the other barrel. And adjust the opposing barrel with it’s diopter, once you’ve done that, both barrels are adjusted equally by the center focus knob. I expect parallax and focus are yoked together in a similar way inside the scope. But are focus and parallax the same thing?

No, you can have a focused image and still have uncorrected parallax or vice-versa. I expect that adding another precisely controlled internal device would grossly increase the price of riflescopes.

Because they can be optically measured and corrected for at the factory, and adjusting the focus of the image and the parallax are similar movements. Coupling them into the same control makes sense. But they aren’t the same thing, they are set and then slaved together to work as one.

With focus and parallax adjusted properly, the target and reticle are clear and stable, as seen through this **US Optics FDN1 7 X**

Last Thoughts

You should be able to identify parallax in your particular setup. Knowing what to look for, and how to correct it is an important part of long-range shooting. Hopefully it makes a little more sense to you, and you can apply it to your shooting for improved results. Remember; Little things matter at a distance.