One of the features we routinely include when we review rifle scopes is parallax and parallax adjustments. Many shooters, even some experts, aren’t entirely sure what parallax is or how it works. This article is meant to give anyone a basic understanding of parallax in rifle scopes.
Parallax in a rifle scope is the result of the image created in the riflescope when the image of the target is not on the same optical plane as the reticle. This out-of-focus condition causes the crosshairs of your scope to seem to wander or move off your target when you move your head or eyes even slightly while you are aiming.
The errors induced by parallax when you are aiming through a scope are not significant enough to be noticeable unless you are using high magnification. Typically, parallax errors become a problem when using 8X magnification or above. It is important for anyone using a high-power scope to understand parallax, how it affects your shooting, and how it is corrected.
Understanding How Rifle Scopes Work
To fully understand parallax, you need a working knowledge of riflescopes and how their internal structures are organized and work together. Before we get to parallax, let’s look at a typical telescopic riflescope.
The Guts of Your Rifle Scope
The usual arrangement inside your riflescope has four lens assemblies
- The Objective Lens
- The Focus Lens
- A Magnifying Lens
- An Ocular Lens
Where these lenses are inside the scope is important. The ocular lens is the lens through which you look to sight your scope. The objective lens is the glass closest to your target and is responsible for gathering all the light you see through your scope.
The last two lenses, the focus lens and the magnifying lens, are housed in the erector tube. The erector tube is the center of the scope and is often smaller in diameter than both the objective lens and ocular lens housings.
First focal plane scopes put the reticle in front of the magnifying lens. A second focal plane scope places the reticle behind the magnifying lens. This helps explain why the reticle changes size in a first focal plane scope and remains static in a second focal plane scope
Understanding the internal arrangement of the lenses and the reticle helps explain the parallax issue.
How All the Lenses Work Together
With a basic understanding of how the lenses and the reticle are arranged inside your rifle scope, we can begin to grasp how parallax fits into this picture. We know that the parallax problem is caused by the image of the target not being in focus, or on the same focal plane, as the reticle. Both images may seem crisp and clear, but this difference is critical to the riflescope’s performance.
Riflescopes with parallax adjustment come in two styles.
- Side Focus Adjustment
- Adjustable Objective
As you might guess, the method of adjusting parallax errors depends on adjusting the internal focus of the target image in relation to the reticle. The key issue is which lens inside the riflescope is moved to change the focal plane of the target image.
Side focus adjustable riflescopes change the position of the focus lens assembly to bring the target image closer or further away from the objective lens assembly. Side focus adjustment knobs are typically graduated in distances from as little as 10 yards to infinity.
Adjustable objective parallax correction works by moving the objective lens assembly in the erector tube. Much the same as a side focus adjustment, moving the objective lens closer or further away from the target changes the focal plane of the image inside the scope housing.
The Problem You May Have Never Heard About
Parallax is a problem that many shooters have never known about. Unless you routinely shoot at long ranges or you are a precision shooter, parallax probably doesn’t figure into your shooting style much. If you shoot at most normal ranges up to 250 yards, parallax error is not a significant problem. There are several reasons that parallax is not a problem for average shooters and hunters.
- Factory parallax settings – Riflescopes all have a sweet spot. The sweet spot is that distance where the riflescope is parallax-free. At this point, the image of the target is on the same focal plane as the reticle. At this distance, no matter where you move your head or eyes, the crosshairs will remain on your target. The sweet spot on most rifle scopes is 100 or 150 yards.
- Average shooters and their most used riflescopes – Most shooters and hunters never shoot further than 250 yards. The favorite scope among most hunters is a 3-9×40 rifle scope. At these distances and using scopes of this power, parallax is not a significant issue. The errors induced by parallax at these magnifications and these distances are rarely noticed.
- The type of riflescope makes a difference – Many types of riflescopes are advertised as parallax-free. Most of these types of aiming devices are either reflex scopes or red-dot sights. These types of sighting tools rarely have magnification. Don’t expect a red-dot sight or reflex style of riflescope to have a parallax adjustment as a standard feature.
Which Type of Parallax Adjustment is Best?
That is a loaded question. Ask it among dedicated long-distance or precision shooters, and you better bring a lunch because the discussion and arguments are going to last a long time. Each side has its opinions and will defend them at length. In brief, both types of parallax adjustment have advantages and disadvantages. When choosing which style of scope is best for you, your needs, expectations, and goals should be the determining factors.
However, before you make that decision, an understanding of these advantages and disadvantages should be clear.
Adjustable Objective Parallax Correction – The Advantages
- AO adjustable scopes can usually remove any parallax error induced by your scopes design. This gives you a broader range of adjustments and often works to the very limits of your scope’s capabilities.
- Your position on your rifle is not as critical with AO adjustable scopes. There is not so much need to have the exact same cheek weld and eye relief distance for each shot.
- The adjustments to AO-equipped scopes are usually made with a ring on the objective end of the scope. This eliminates having another turret or other adjustment knob on the side of the scope.
- With practice, the AO adjustable scope can be used as a range finding tool. However, you must understand that this takes practice and is, at best, a rough estimate.
- Scopes built to use an AO adjustable parallax system tend to be more durable and more rugged than side-focus scopes.
- Most AO adjustable scopes use a threaded adjustment mechanism that allows finer adjustments
Disadvantages of Adjustable Objective Riflescopes
- AO riflescopes are usually more expensive than fixed objective lens scopes.
- The mechanism used to manipulate and move the objective lens assembly adds additional weight to the riflescope.
- Many shooters find it necessary to move from their aiming position to effectively manipulate the AO adjustment rings.
- The deep depth of field that comes with AO-equipped scopes can lead to problems of misplaced shots for some shooters.
Side Focus Riflescopes – The Advantages
- Side focus parallax adjustment rifle scopes are usually easier to operate than AO-equipped scopes. Most shooters can manipulate and adjust a side focus equipped scope without moving from their aiming position.
- Side focus scoped tend to have a shallower field of view than AO scopes. This shallower field of view can be an aid in adjusting for wind or other factors.
- The arrangement of the side focus turret makes these scopes easier to use in the field.
- The side focus parallax adjustment system is less critical of cheek weld and head placement than AO scopes.
Disadvantages of a Side Focus Parallax Adjustment
- Side focus adjustments typically take more time to make than those on an AO-equipped riflescope.
- The internal design of most side focus adjustment systems makes finer tuning almost impossible. The turret-based adjustments depend on “clicks” for each movement. A single “click” may impart more adjustment than is needed leaving you without a precise adjustment point.
- Side focus parallax adjustments may impart some blurriness to your target image.
- The side focus adjustment turret is subject to the same kind of damage as the other turrets on the scope.
Do I Need a Scope with Parallax Adjustment?
This is a very personal question and one that doesn’t have a clear black and white answer. Several issues need to be considered before you make the decision to spend the extra money to purchase a scope that allows for parallax adjustments.
- What kind of shooting do you do? – The type of shooting you normally perform is perhaps the biggest indicator of your need for parallax adjustment on your scope. The average target shooter or hunter rarely engages targets beyond the 250-yard distance. At these ranges, most scopes have so little parallax error that trying to make parallax adjustments becomes a moot point.
- How willing are you to learn how to use the adjustments correctly? – Learning to use the parallax adjustment on any scopes has a learning curve attached. Becoming proficient with making parallax adjustments takes time and practice. Making parallax adjustments is not a simple click-and-shoot situation.
- Is the extra cost a factor? – Adding either type of parallax adjustments to a riflescope imparts extra cost. The mechanisms used to correct for parallax errors must be precise and rugged at the same time. Such equipment adds both cost and weight to the scope.
- What magnification scope do you anticipate purchasing? – Most hunters and shooters opt for telescopic riflescopes that rarely go above 8X magnification. At these magnifications, parallax errors are small and don’t affect most shooters appreciably. If you anticipate buying a scope with magnification settings above 8X and think you will routinely use these higher magnifications, a scope with parallax adjustments may well be a good idea.
To Adjust or Not Adjust. What are the Benefits?
The good and bad of having parallax adjustability on your file scope can be the deciding factors for your purchase. In summary, the benefits of scopes with parallax adjustment include:
- Increased accuracy, especially at high magnification or extreme distances
- Better accuracy means more kills and more one-shot kills at longer distances
- Adds versatility to your rifle scope
- Can make adjusting for other factors such as wind and bullet drop easier
- Can reduce the need for precise positioning on the rifle with each shot to maintain precision
There are some disadvantages to a riflescope with parallax adjustments that should be considered.
- Parallax adjustments are another complication to your scope. This means more adjustments and more internal equipment that can break, leaving your scope inoperable in the field.
- The additional parts needed to affect the parallax adjustments add weight to the rifle scope. For many long-range hunters who spend hours trekking over rugged terrain, even a few ounces of extra weight are a serious consideration.
- Cost is a huge factor for some shooters. A comparable fixed scope without parallax adjustment is usually significantly cheaper than the same scope with magnification and parallax adjustments.
My Take on Parallax Adjustable Scopes
For me, it boils down to a single factor. Money is usually the tipping point in these decisions. If I can afford the difference in cost, I will always choose a scope with parallax adjustment. Why? There are several reasons.
- I would rather have it if I need it. – It is always better to have the option if you need it than to find yourself in a situation where you need to adjust for parallax, and your scope doesn’t have the feature. Just because your scope has parallax adjustments doesn’t mean you have to use them for every shot
- The older I get, the more help I need. – I intend to keep shooting and hunting for many years. However, as I age, my eyesight needs more and more help. Having the versatility of parallax adjustment gives me an edge on longer shots at higher magnification, especially when my eyes aren’t helping the situation.
- I like to buy quality and precision. – My opinion is to buy the best of anything for the long haul. I don’t change scopes on a seasonal basis like some hunters and shooters. Many of my rifles still have the first scope I bought mounted. The scopes I chose were the best I could find for the budget I had, and they still perform their jobs to my needs.
In the End, Finding Your Comfort Zone is the Key
Purchasing your riflescope, be it with or without parallax adjustments, is like finding the best boots for your feet. Comfort and fit must be correct, and the choice must meet the uses you anticipate.
I hope this article has given you a better understanding of parallax in riflescopes and helps you make the best decision possible as you shop for your next riflescope. If you have thoughts, ideas, or advice, please share them in the comments section below. We all benefit from your wisdom and experience.