How Does A Scope Work | The Basics Of Riflescopes [Explained]

Most shooters have a basic understanding of how the scope mounted on top of their rifle works, but surprisingly few are aware of the actual science that goes into crafting a high quality rifle scope.

Don’t worry, we aren’t going to give you an advanced physics lesson – that’s a bit outside of the scope of this article (no pun intended). But we will give you a basic breakdown on the mechanics behind your optic and how understanding it all can help improve your shooting.

Why Does It Matter?

If your answer to the question "how does a scope work?" is "why should I care?," consider this: understanding the interplay between your scope and your rifle can have a measurable impact on your effectiveness as a shooter, and can make it much simpler and less frustrating to get a new scope zeroed in or adjusted in the field. 

It can also help you separate the wheat from the chaff when it comes time to buying a new optic – there’s nothing worse than throwing money away on a bad scope because you didn’t know in advance what kind of features and performance you were looking for.

At the end of the day, if you want to save time and money while becoming a better, more knowledgeable marksman, a basic understanding of the tools at your disposal is essential.

How Does Scope Magnification Work?

Ever wonder how rifle scopes actually magnify the image that you see when you look through the lens? It may surprise you to learn that a rifle scope works in much the same way as the telescopes you may have used to do some stargazing when you were younger. Many of the same basic components are the same, from the objective lens and ocular lens to the erector lens and magnifying lens - though of course the magnification range is not nearly as high!

First, light passes through the objective lens at the front of the scope and meets at the focal point. The bigger the objective lens, the more light is let into the scope, which results in brighter, more high contrast images and better low light performance, at the cost of a larger and heavier scope profile.

At this point, the image is inverted, and without an erector lens it would be very difficult to make an accurate shot. But as it passes through the erector lens, it is flipped to the proper orientation and sent through the magnification lens, which, in a variable power scope, increases or decreases the size of the image based on how close it is to the ocular lens (the rear lens that you actually look through).

For example, on a 3-9x scope, setting the scope's magnification to 9x will pull the magnification lens closer to the ocular lens, making the image that you see through the scope appear larger.

On fixed power scopes, the magnifying lens is set in a fixed position and cannot be moved, which is why this type of rifle scope is typically thinner and more compact than most variable power scopes.

If that wasn’t complicated enough, there is also the distinction between a first focal plane reticle and a second focal plane reticle. In an FFP rifle scope, the crosshair is mounted on the front of the magnification lens, causing the crosshair to change sizes as the magnification settings are adjusted in order to maintain the same relative size when aiming at the target.

In an SFP scope, the crosshair is mounted at the back of the magnification lens, which results in the reticle staying the same size regardless of magnification settings.

Both first and second focal plane scopes have their fans and detractors, so it's worth trying out both to see which one you prefer. To generalize a bit, SFP scopes see more use when time isn't an issue and the shooter has plenty of time to make elevation and windage adjustments, while FFP scopes tend to be favored by those who need speed and simplicity, like a target shooter at a timed competition.

What Do The Two Knobs On A Scope Do?

On the main body tube of your scope you will see two (possibly three, but we’ll get to that in a minute) knobs, typically oriented on the top and right side of the tube. These are your target turrets, and they allow the shooter to make adjustments to elevation and windage in order to ensure that your aiming point and point of impact are properly lined up.

For hunting scopes, you may have to remove protective turret caps before making adjustments, while most scopes designed for long range shooting or tactical use leave the turrets bare to facilitate faster adjustments in the field.

Each knob should have directional indicators, as well as display the “click value,” which tells you how much each click of the turret will cause your reticle to move.

How Do Scope Adjustments Work?

Adjusting a modern scope consists of finding the perfect balance between three factors: windage, elevation, and parallax.

The two adjustment turrets that you will find on every single scope tube are windage and elevation, which allow the reticle to be moved horizontally and vertically. In fact, when you are making scope adjustments, the reticle is the only thing that moves at all, which is why it is highly recommended to zero in a scope while shooting from a stable platform such as a rifle rest, sandbags, or even a shooting vice.

The scope’s adjustments are measured in either minutes of angle (MOA) or miliradians (MIL) – the former is much more common, and much more intuitive to understand. The majority of modern scopes will have a click value of 1/4th MOA, meaning that each time you hear a click while turning the knob, the reticle is being adjusted by 1/4th MOA in that direction.

As a brief example, imagine you are sighting in a scope at the recommended distance of 100 yards and your shot is dead center, but 2 inches high. That means you need to adjust the reticle downward 2 by MOA, and since the click value is 1/4th MOA, that means turning the adjustment knob until it clicks 8 times. If your scope had a click value of 1/8th MOA instead, you would need to turn the knob until it clicked 16 times.

These adjustments work the same way for both windage and elevation adjustments, and while it may take a bit of practice, you’ll only need to make the adjustments once unless you decide to zero the scope at a different distance later on.

Even then, many rifle scope manufacturers offer scopes that come equipped with zero reset turrets which allow you to quickly and easily return to your original zero after making adjustments, and the increasing popularity of the BDC reticle design makes it even less likely that you’ll need to be constantly fiddling with adjustment knobs.

Finally, there’s parallax adjustment. Parallax error occurs when the image of your target projected through the scope does not appear on the same optical plane as your scope’s reticle.

In other words, what this means is that the target image or reticle may appear to shift when you move your eye away from the exact center of the eyepiece, though this effect can also appear in the form of a hazy or “blacked out” border around the lens when looking through the scope.

Parallax is almost never an issue at close range, and most rifle scopes with lower magnification settings do not include a parallax knob. However, high-end optics meant for long range shooting typically do, and are often listed as “side focus” or “side parallax” scopes.

Using the parallax knob isn’t much different from the windage or elevation knob – make sure you’ve got a crisp, clear image, aim dead center at your target, and make small adjustments to the parallax knob while slightly shifting the position of your head. When you stop noticing any movement of the reticle in relation to your target, you’ll know you’ve got your parallax correctly dialed in.

In addition to these primary adjustments, your scope will likely also have a target focus ring at the rear of the scope (often around the ocular lens) that ensures a crisp image at different distances, and a power ring to increase or decrease the scope magnification levels, which is usually located in front of the ocular lens, but may rarely be incorporated directly into the objective bell as well.

How Do Scopes Line Up?

You might be wondering how exactly a scope can accurately match a round’s point of impact when the optic is mounted so much higher than the bore axis of the rifle – in some cases, by several inches. The answer to this involves a few different factors:

First, it’s important to remember that gravity starts pulling the round downward as soon as it leaves the barrel of the rifle, which is commonly known as bullet drop. That means that the farther away a target is, the higher the rifle’s muzzle will need to be angled in order for the round to land accurately, creating a curved trajectory.

Second, we must keep in mind that the line of sight is a perfectly straight line from the shooter’s naked eye to the target. A rifle scope doesn’t change this – when looking through a scope, the line of sight is still a straight line.

So when a scope is mounted to your rifle, the goal is to make the line of sight and the bullet’s trajectory intersect at the exact distance your target is located. To do this, the scope’s reticle and the angle of the muzzle work together to compensate for one another. For example, if you adjust your reticle downward, you will have to raise the muzzle of the rifle in order to properly line up your shot. 

Because your rifle will always require an upward angled muzzle to account for drop, the bullet appears to “climb” after leaving the barrel, rises above the line of sight, then begins to drop again, until it finally intersects with the line of sight for a second time – exactly where your target is sitting.

This is also why rifle scopes must be zeroed in at specific distances, though the most popular zeroing distances allow enough leeway to still make accurate shots at slightly longer or shorter ranges where the bullet’s trajectory has not significantly changed.

Are Scopes Accurate?

While a good scope provides the shooter with a larger, clearer view of the target than the naked eye is capable of, it is important to remember that an optic only improves the shooter’s ability to make precisely aimed shots – it does not affect the inherent mechanical accuracy of the rifle itself.

A high quality scope can be called “accurate” if all of its components and other features are functioning properly and as advertised. Does the scope allow for parallax adjustment? Are the click values of the windage and elevation knobs correct, and does the zero hold without drifting? If so, the scope itself is accurate, and the rest comes down to the mechanical accuracy of the rifle and the ability of the shooter.

Many modern rifle scopes also include BDC, or bullet drop compensation, markings that allow the shooter to make holdover adjustments on the fly without needing to adjust the optic. These reticles are popular among both target shooters and hunters for their added versatility, and they are becoming increasingly more available at very affordable price points.

While these markings are usually quite accurate, keep in mind that even these rely on having an accurate estimation of the target’s distance – even if your rifle scope is using a perfectly accurate BDC holdover meant for 250 yards, the shot will still miss if the target is actually 400 yards away.

There are also dozens of other factors that can impact accuracy, ranging from weather conditions and the consistency of ammunition tolerances to barrel wear or a warped rifle stock. These factors may not be noticeable at 50 yards, but at longer distances they start to add up.

One of the most important factors affecting scope accuracy is something that many shooters overlook: the mount. A quality pair of mounting rings can make all the difference when it comes to holding a proper zero, and even the best scope in the world won't perform if your scope rings are wobbling around.

Parting Shots

While the title of this article may have sounded a little daunting, understanding how your scope works in tandem with your rifle is not as complicated as it seems at first glance. And once you’ve spent a bit of time using this knowledge on the range, you’ll start to realize that making adjustments and sighting in rifle scopes has become second nature.

If this article has you looking for a new rifle scope, check out our guide on the best rifle scopes under $1000! 

And as always, if you found this helpful or just have something to add, sound off in the comments and let us know what you think. We love hearing from our readers, and your feedback helps us choose what topics to cover next.