If you are a shooter or hunter, you may have noticed that the reticle on your scope appears to move in relation to your target when you move your head. If you have, you are experiencing parallax on your scope.
This phenomenon is common on all optical equipment, but is more noticeable in rifle scopes. Parallax can be a problem, especially for long-range shooting.
Parallax is an optical illusion that occurs when the reticle that appears in your scope is in a different focal plane than the target image. This difference in the focal plane causes the reticle image to seem to float above or behind the target image, causing the reticle to move when you move your head.
When the parallax adjustment is made correctly, the reticle appears to be fixed on your target image.
Not all scopes feature parallax adjustment as a feature. Depending on the scope’s power, many scopes have a fixed parallax setting, usually at 100 or 200 yards. On low-power scopes, parallax is not a big issue and doesn’t come into play. However, parallax adjustment systems are needed to make your scope useable and accurate if you routinely use higher power magnification and shoot at distances over 200 yards.
What Causes Rifle Scope Parallax Error?
Rifle scopes are much more complex pieces of equipment than most shooters realize. A rifle scope does many jobs, including:
- Gathering light from the target image
- Intensifying the image
- Correcting any inversions or distortions
- Inserting the reticle image
- Magnifying and focusing the image before transmitting it to the shooter’s eye.
When you are looking through the scope, you see an image made up of several different layers.
To see everything in the proper relation, the images must all be on the same focal plane with the same focal point. To eliminate parallax, the scope must be capable of adjusting parallax error by bringing the projected image of both the reticle and the target into the same plane.
How is Rifle Scope Parallax Adjusted?
How the adjustments to correct parallax errors in your rifle scope are made depends greatly on how the rifle scope is constructed.
In general, rifle scopes built to allow parallax adjustments have a third turret or knob on the offside of the rifle scope. In many cases, the knobs are marked and graduated in increments between 50 and 100 yards per click. Some better rifle scopes allow parallax corrections in smaller yardage increments.
First Focal Plane vs. Second Focal Plane Rifle Scopes
If your rifle scope is a first focal plane design, the reticle is projected at the front of the scope system, usually on the first lens behind the objective lens. First focal plane scopes keep the reticle’s size in proportion to the magnification of the target. In essence, the reticle gets larger as the magnification increases.
A second focal plane scope puts the reticle at the rear of the erector assembly behind the magnification system. On a second focal plane scope, the reticle remains static as you dial the magnification up or down.
The downside is you must remember that as magnification increases, the hash mark measurements on your require mental compensation.
Parallax Adjustment Systems
There are two primary ways rifle scope parallax adjustment systems work: the scope may have an adjustable objective lens, or it will have a side focus assembly that allows you to compensate for parallax errors.
In either case, parallax adjustments are much the same. You turn the parallax dial or turret on your scope to the correct distance measurement, and the system adjusts the erector or objective lens for a parallax-free image in your scope.
Are Parallax Issues Really a Problem?
For the average hunter shooting at targets under 200 yards, parallax issues are usually not readily apparent in their rifle scopes. Most scope manufacturers set their scopes to adjust for parallax at a given distance.
Long-range shooting presents some different requirements and problems with parallax issues that need compensation for accurate shooting.
Many shooters who engage targets at long distances need to be able to adjust the target and reticle into the same focal plane for precise and accurate shooting. Any movement of the reticle on the target at ranges greater than 200 yards can significantly alter the aiming point of a scope.
Which Parallax Adjustment System is Better?
Deciding which type of parallax adjustment system is better is a very personal choice.
Each adjustment system has its advantages and disadvantages that must be considered. Understanding each system is critical, especially when you are purchasing a high-power scope for your rifle.
Adjustable Objective (AO) Systems
Adjustable objective lens systems deal with parallax issues by moving the objective lens to change the focal point of the images to bring the reticle and target into the same focal planes.
This is usually done with an adjustment ring at the front of the rifle scope. This eliminates having a third adjustment turret at the middle of the scope body. If you are considering an adjustable objective system on your scope, you should consider these pros and cons:
- Often offer greater depth of focus than other systems
- AO scopes typically have finer adjustments than side focus systems
- No extra turrets or knobs on the scope to be damaged
- May be more resistant to damage than a side-focus scope
- It is often able to be used as a rudimentary range finder
- Usually more expensive than side focus or fixed objective scopes
- Sometimes heavier than other scopes
- It may not be as easy to adjust while in a shooting position
Overall, I prefer an adjustable objective system for parallax adjustments to a side focus system. Eliminating the extra turret to the side of the scope makes for a cleaner and more functional scope.
Side Focus Systems
The side focus system uses internal adjustments to the erector system in the rifle scope to adjust the reticle to the same focal plane as the target image. This usually requires a third knob or turret on the offside of the rifle scope to make these adjustments.
Typically, the turret is graduated in measurements of 50 or 100-yard increments that can allow quick adjustments to the parallax settings. In some cases, the parallax adjustment turret also serves as the intensity level adjustment for scopes with an illuminated reticle.
- Easy to use while hunting or shooting
- Allows the shooter to see the adjustments without leaving a shooting position
- Often combined with reticle illumination adjustments for convenience
- It may offer a shallower depth of focus that can make wind readings easier to estimate
- Are often lighter than AO systems
- It doesn’t offer the fine-tuning capability of an AO system
- This may cause some image blurring as the parallax is adjusted
- Adding an additional turret to the scope can be another point of damage or failure during use
I find side focus systems easier for some shooters to use and understand than an AO system. Each click represents a fixed distance. If range estimation is done properly, this can be an efficient system.
Coming to Grips With Parallax Error
The average hunter or target shooter may never encounter parallax error that affects their hunting or shooting.
Modern rifle scopes built with pre-set parallax adjustments for the most common distances make these problems almost non-existent. However, if you go much beyond 200 yards with your shooting, you will eventually encounter problems with parallax error.
In these cases, a scope with parallax adjustment features becomes much more important to the success of your shooting.