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How Does a Rifle Scope Work?

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Optics is one of the fastest-growing sectors of the firearms world, empowering marksmen to hit their targets faster and more reliably from almost any distance. Once relegated to specialists, rifle scopes are now commonplace for rifles of all kinds, including those meant for hunting, competition, and tactical/defensive use.

So, how exactly do rifle scopes work, and what advantages can they bring to the user? In this article, we’ll cover the basic mechanics of rifle scopes while detailing their core functions and features. We’ll also discuss how modern rifle scopes have evolved, contributing to their meteoric rise in usage and innovation.

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What is a Rifle Scope?

A rifle scope is a telescopic firearm optic that uses a series of glass lenses to display an aiming reticle over the target. Today’s rifle scopes come in many shapes and sizes, offering fixed or variable image magnification. That said, most of today’s rifle scopes have variable magnification, allowing users to adjust their zoom through a defined range (1-8x, 4-16x, 6-30x, etc.)

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Generally, a marksman will choose a scope with a magnification range optimized for the rifle’s intended and effective range. For example, a long-range marksman shooting over a kilometer would choose a scope with high magnification like 6-30x, allowing them to see and engage those distant targets.

However, that same 6-30x rifle scope would be a poor fit on a defensive carbine, which would be used mostly within 50 yards. The scope would be too cumbersome, and the high magnification would be extremely disorienting when attempting to aim at short distances. Instead, you would want a 1-6x or 1-8x rifle scope, which offers the flexibility of a low-power 1x setting for close-quarters agility.

To know the best application of any given rifle scope, you’ll need to understand the ‘anatomy’ of a rifle scope. This will allow you to determine what ranges and environments a scope will excel in, making it easier to choose the best scopes for your own rifles.

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1. Objective Lens

Your objective lens is the big lens up front. The main purpose of the objective lens is to collect light. Larger objective lenses collect more light, creating a brighter and larger image, but it comes at the cost of optic weight and size, as larger objective lenses require a longer focal length to maintain image quality and depth of field. That’s why optical engineers must carefully balance a scope’s magnification range, objective lens diameter, and overall scope size to deliver the best performance for the scope’s intended niche.

You can always determine a scope’s objective lens diameter by its name. Most scopes follow the same format, which tells you the minimum magnification, maximum magnification, and objective diameter. Thus, you can read a ‘1-8×24’ scope as having 1-8x variable magnification with a 24 mm objective lens diameter.

One common misconception is that a bigger objective lens means a bigger field of view. The field of view is the amount of the observable world that you can see while looking through the scope. For rifle scopes, the field of view is usually expressed as a width (measured in feet) at 100 yards. Variable rifle scopes will provide two measurements: field of view at minimum magnification and field of view at maximum magnification. This is because higher magnifications appear closer, which necessarily reduces the world’s observable width in the image.

Ideally, your scope should have a large field of view, making it easier to see everything around your target for greater awareness, but it is not as simple as having a large objective lens. Field of view is derived from a multitude of factors, relying more upon the shape of the lenses rather than just diameter. Scope manufacturers will design their lenses to offer the maximum field of view possible without causing imperfections and distortion. This is one reason why high-quality rifle scopes are often distinguished by their edge clarity.

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2. Scope Body

As light passes into the objective lens, it is focused into the scope body tube. Scope bodies come in several diameters (1-inch, 30 mm, 34 mm, etc.), and it’s important to know the diameter of the scope body when mounting the optic, as mounts need to be sized to match. The main benefit of a wider scope body is a larger elevation adjustment range, which we will describe later. The trade-off is that larger bodies make for heavier optics, and you may not need that extra elevation adjustment if the scope is used at short distances.

The scope body contains two major internal assemblies: the Focus Lens Assembly and the Erector Tube Assembly.

The Focus Lens Assembly serves the vital function of correcting parallax—the displacement that occurs when observing an object from different angles. In rifle scopes, parallax can cause an inconsistency between your apparent and actual point of aim, so it’s essential to control parallax in long-range shooting.

Depending on the design of the rifle scope, the Focus Lens Assembly may be set to a defined distance (often 100 yards) or adjustable via a knob on the side of the scope. Generally, you want parallax adjustment for any high-magnification precision scope, while fixed parallax is better for low-magnification rifle scopes.

Behind the Focus Lens Assembly, the Erector Tube Assembly contains your magnifying lenses and your scope’s reticle. It’s called the ‘Erector’ because it translates your image into its proper ‘upright’ view. Without it, the scope’s view would appear upside down!

The magnifying lenses inside the Erector Tube Assembly determine the apparent magnification of your image. As you increase your scope’s magnification, these magnifying lenses move toward the objective lens. As you reduce magnification, they move back towards the eyepiece.

Your reticle will be in front of or behind the magnifying lenses, depending on whether you have a first focal plane (FFP) or second focal plane (SFP) scope. Because the first focal plane reticles are in front of the magnifying lenses, these reticles scale correctly with magnification, allowing you to use reticle subtensions at any power. On the other hand, second focal plane (SFP) reticles do not scale with magnification and are only accurate at a single magnification—usually maximum.

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FFP reticles are usually more expensive than SFP reticles, but we highly recommend them for any scope where you may need to use reticle stadia at a magnification setting below maximum.

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3. Elevation and Windage Turrets

Located on the top and right sides of your rifle scope, the Elevation Turret and Windage Turret control the up/down and right/left movement of your Erector Tube Assembly.

Turrets allow you to match or “zero” your rifle scope’s reticle with your rifle’s point of impact. Zeroing can be a challenge for new marksmen, but we’ve detailed some helpful techniques in another article.

To zero effectively, you’ll need to know whether your scope’s turrets are MIL or MOA. Turret adjustments are tiny, so consistency is essential—especially when making precise shots at long distances. That’s why turrets ‘click’ when you adjust them, as they limit your reticle’s movements to consistent intervals, measured in Milliradians (MIL) or Minutes of Angle (MOA). Neither measurement is superior, per se, but they offer different advantages, which we detail in this article.

It is also important to know how your turrets are meant to be adjusted. Elevation and Windage turrets come in two designs: exposed and capped. Exposed turrets are enlarged knobs designed for precise manipulation in long-range shooting. Capped turrets are much smaller, meant to be ‘set and forget’ with threaded caps that prevent them from turning unintentionally.

Generally, you will see exposed turrets on precision-oriented, high-magnification scopes, while capped turrets are more common on low-magnification carbine scopes. Either way, when mounting your optic, make sure your mounting rings do not contact or block the turrets.

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4. Illumination Knob

If your scope features an illuminated reticle, it will probably have an illumination knob. By turning this knob, you can increase or decrease the brightness of your reticle illumination, ensuring optimal visibility without washing out the target. In most cases, this knob will also house the battery for your reticle illumination.

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5. Parallax Adjustment

If your scope has adjustable parallax, it will have a control knob somewhere on the scope body. Sometimes, parallax adjustment appears alongside the illumination controls in order to save space.

The parallax adjustment knob has visible range numbers on its surface, allowing you to quickly adjust to the appropriate setting for your target’s distance. The maximum ‘infinity’ setting is used at extreme distances.

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6. Magnification Ring

Located at the front of the eyepiece, the magnification ring controls the apparent zoom setting of the scope. Like your parallax adjustment, the magnification ring should have visible numbers, allowing you to find the best power for any distance.

Nowadays, many rifle scopes can accommodate a ‘magnification lever.’ A magnification lever is an extended post attached to the magnification ring, giving the user more leverage when making adjustments. Magnification levers are an excellent feature for competition or tactical rifle scopes, improving your speed when moving between targets.

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7. Ocular Lens

The rearmost component of your scope’s anatomy is the eyepiece and ocular lens, which emit the visible image. This part of the scope also features your diopter adjustment, a small ring that allows you to adjust the reticle’s focus. The diopter should be set to accommodate your own specific eyesight. Most scope manuals will include details on how to do this.

The visible image comes from the center of your ocular lens. The dimensions of the image are measured by length (Eye Relief) and diameter (Exit Pupil). Eye Relief and Exit Pupil make up your usable ‘eye box.’ To look through the scope, you must center your pupil within this eye box, or you will see scope shadowing and a partial image. A larger eye box equates to an optic that is more forgiving to head and eye placement.

Eye Relief and Exit Pupil are a product of magnification, objective diameter, and internal optical design. Generally, it’s better to have a larger eye box, especially in low-magnification scopes. That is because low-power rifle scopes must perform well in close quarters, where fast target acquisition is critical.

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How Does a Rifle Scope Magnify Images?

Rifle scopes magnify images by passing light through a series of lenses arranged in a specific order. The objective lens, located at the front of the rifle scope, is responsible for gathering light and forming an image of the target. This image is then projected onto the erector assembly, which uses a series of lenses to overlay the reticle onto the target image, magnify the image, and flip the image. It then sends the image to the third lens, the ocular lens, which emits it to your eye.

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The amount of magnification that a rifle scope provides is determined by the shape and position of the magnifying lenses inside the erector assembly. A scope with a magnification of 4x means that the image will appear four times closer than it would to the naked eye. Similarly, a scope with a magnification of 10x will make the image appear ten times closer.

For scopes with variable magnification, you can turn the magnification ring on the ocular bell to change the zoom setting. Internally, this is moving the position of the magnifying lenses within the scope.

It is important to note that the quality of the lenses, the construction of the scope, and the lens coatings used in a rifle scope can significantly affect the clarity and sharpness of the image. High-quality lenses and coatings will provide a clearer and brighter image, while lower-quality lenses and coatings may result in a blurry or distorted image.

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How Does a Rifle Scope Adjust for Parallax?

When properly used, a rifle scope can be precise at a great distance, but it can also be compromised by something called parallax.

Parallax appears when the reticle, or crosshairs, seems to move in relation to the target when the marksman moves their head. At extreme distances, this can cause the marksman to miss their target entirely.

Fortunately, most precision rifle scopes have a parallax adjustment feature that allows you to correct this issue. The adjustment mechanism typically consists of a knob or dial located on the side of the scope, allowing you to adjust the focal position of the reticle in relation to the target by repositioning the focal lens assembly.

In most cases, your parallax adjustment will have clear numbering on the side, ranging from a minimum adjustment to ‘infinity,’ often listed in 100-yard intervals. If you know your distance to the target, this makes it easy to set the parallax adjustment. If you do not know your distance to the target, use a laser range finder or your reticle to find the distance to the target.

If you do not see a parallax adjustment knob, the scope probably has a fixed, non-adjustable parallax. In this case, refer to the scope manufacturer’s specifications to determine the calibrated distance.

Ultimately, parallax adjustment is an essential feature for precision-oriented rifle scopes. However, it may not be necessary for low-power rifle scopes, which are best at distances where parallax error isn’t a major factor.

primary arms scope mil bdc holdover reticle down range view zoom

How Does a Rifle Scope Reticle Work?

A rifle scope reticle describes the lines and markings that appear within the visible image as you look through the optic. Your reticle provides a clear point of aim, as well as clearly marked stadia which can help you make measurements, range estimations, and ballistic holdovers.

There are many different types of reticles. They can be as simple as a standard crosshair or as comprehensive as a full MIL grid. Like scopes themselves, reticles have become more advanced over time, incorporating new technologies and mathematics to improve the speed of the marksman.

The ever-popular Duplex and MIL-DOT Reticles are still common, but most tactical and long-range enthusiasts are looking for more than featureless crosshairs and hashes.

Today’s most advanced reticles fall into three main categories: Minute of Angle (MOA), Milliradian (MIL), and Bullet Drop Compensator (BDC).

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MOA scopes and MIL scopes are very similar. Both have stadia markers that occur at evenly spaced, numbered intervals. The only difference is the size of those intervals, as MOA and MIL are different units of angular measurement, much like degrees. MOA and MIL scopes are most popular for long-range precision, where the marksman will make small adjustments with their turrets to correct for extreme distances, though there are many close-quarters reticles with MIL subtensions as well.

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BDC reticles, such as our original ACSS® reticle, do not use a fixed unit of measure. These scopes have markings that are calibrated to show the trajectory of the bullet itself. To make a BDC reticle work, you’ll need to verify that your rifle and ammo line up with the BDC reticle’s holdovers. In most cases, BDC scopes come with specific instructions for which cartridges and rifle setups will be compatible, often including zeroing recommendations to help you get the best results.

BDC reticles are used in a wide variety of tactical rifle scopes, but they are also increasingly popular for long-range competition and hunting as well. BDC reticles offer faster acquisition and engagement speeds when compared to MIL and MOA scopes, which require memorization or mathematics to calculate holdovers. MIL and MOA reticles will work with any rifle or ammo, though, so if you change your setups frequently, or use a less common specialized cartridge, these reticles are slightly more versatile.

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How Does a Rifle Scope Account for Distance and Windage?

You can use a scope to account for ballistic drop and windage in two ways: turret adjustment and holds.

Turret adjustments are self-explanatory. Once you’ve calculated a correction for ballistic drop or for wind, you can adjust your turrets to ‘zero’ your center aiming point on the new point of impact that accounts for your wind hold. This practice is usually only done with scopes that have exposed turrets—predominately precision-oriented scopes with magnifications of 10x or more.

You can also use your reticle to ‘hold’ for corrections, essentially relying on the reticle’s stadia instead of making physical adjustments to the scope turrets. For BDC reticles, your ballistic holdover and wind corrections may already be pre-calculated and marked in the optic. For MIL and MOA reticles, you will need the correct drop and wind calculations, which can be derived by a ballistic calculator or an established ‘Data on Previous Engagements’ (DOPE) card.

For long-range precision shooting, most experienced marksmen prefer to adjust for elevation and hold for windage. After all, winds can change on a dime, while gravity does not. Alternatively, if you are using a scope with a BDC reticle, you likely will hold for all your ballistic solutions unless you are engaging a target beyond the reticle’s max calculated distance.

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How Does a Rifle Scope Work in Low Light Conditions?

In low-light conditions, your scope’s capabilities become more evident. High-end rifle scopes are able to perform in much lower light conditions than entry-level models, but no rifle scope can function in total darkness without a bit of help. If it’s getting too dark for your rifle scope, you’ll need a separate night vision or thermal device.

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Clip-on night vision isn’t cheap, nor is it a standalone solution. You’ll likely need additional equipment to illuminate targets, at the very least. But if you are truly set on taking on the night, the technology is available. In fact, we carry a wide range of night vision equipment and essential gear to help you get started. Keep in mind, though, you’ll also need a range that allows after-hours shooting, which can be rare in some parts of the country.

We also have a comprehensive guide on night vision that details all the concepts, tools, and specifications to know.

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Rifle scopes aren’t just your average firearm accessory. They are part of an exciting, emergent technology that continues to improve the capabilities of modern riflemen. As scopes become more and more advanced, we expect to see their potential improve.

Will the future be laser-emitting scopes that automatically correct for your ballistic drop down to the inch? Or will they be thermal-sensing detection tools with lightweight polymer and fully digital images?

It’s difficult to say, as scopes continue to develop and incorporate new technologies in the name of pushing the boundary of precision. Hopefully, this article has given you some more context to this subject, so you can follow along as optics manufacturers continue to evolve their product lines.