How Does a Rifle Scope Work: Optics Anatomy 101

A hunting scope, be that for a handgun, bow, shotgun, or rifle, is used to get the shooter a point of aim on the target. Ideally, this helps the shooter be more accurate. Some scopes even enlarge the scene viewed through the scope. So, how does a rifle scope work?

Up next see our write up on different long range scopes for the 6.5 Creedmoor.

How Does a Scope Work?

Gathers Light

Essentially, one of the main functions of a scope is to gather the light reflected off a target and present the resulting image to your eye. Some scopes can magnify this image, while others can’t. 

The scope uses glass lenses to do this. Even electronic scopes have a glass focus lens to gather and focus the incoming light.

The objective lens, commonly referred to as the “bell,” is located at the front of the scope and is measured in millimeters. Note that a larger objective lens has the capability of gathering more light, thus producing a brighter image.

So, a 55mm bell will gather more light than a 40mm bell. Large bells are an advantage for people who shoot in the twilight hours. (1)

This can extend your hunt by 10-15 minutes, depending on the conditions. As deer hunters know, even 5 minutes can make a difference between going home skunked or going home with something for supper.

The objective lens can be smaller, larger, or the same size as the ocular lens (the lens you look through).

Rifle Scope Terminology

Before we dive in further, here are some common rifle scope terms and definitions to know:

Objective Bell – The front part of the scope that gathers light.

Objective Lens – The frontmost lens in the scope. You cannot make a rifle scope objective lens adjustment as this part is fixed. However, you can change the focus.

Scope Tube – The metal tube that houses all the scope components.

Scope Materials – Scope materials are metal for the tube, glass for the lens, and other metals for the adjustment controls. Some scopes have rubber seals to keep things out of the interior. Differing rifle optics types may have other materials depending on the scope maker and type.

Eyepiece – The rear of the scope, aka the part you look through. This houses the ocular lens.

Reticles – Otherwise known as crosshairs, these are the lines inside the scope that you place over the target.

Scope Mechanics

A scope uses mechanical adjustments to move the reticle, otherwise known as crosshairs. Moving the reticle is important because you want the center of the reticle to cover the spot your bullet hits when you shoot. 

Its mechanics also adjust magnification in unpowered and traditional rifle scopes. Powered scopes may use a mechanical or digital zoom.

Erector System

So, the erector system in a rifle scope refers to the components that create magnification and help adjust for elevation (up and down) and windage (left and right). The adjustments are made with adjustment controls mounted inside turrets in the middle of a traditional scope. Electronic scopes, on the other hand, may have buttons to do this.

To make manual adjustments, look for the windage and elevation knobs on the main tube – it’ll show which direction to turn the knob to go up or down and right or left. A power ring will be present to make adjustments to magnification (if the scope has variable magnification).

Some scopes have an extra knob for fixing parallax error, which is when the reticle moves off target when you move your head. Fixing this helps keep the reticle on target at the distance you want.

How Do Gun Scopes Work if the Scope Is Above the Barrel?

Since the rifle scope sits above the gun barrel, how does the system work, exactly? Well, this is where the understanding of sight planes and the erector system we just discussed comes in.

Looking through the scope to your target is a straight line, a single plane if you remember middle-school geometry. The barrel of the rifle is another single plane.

The complications begin when the bullet leaves the barrel. When the projectile clears the muzzle, it starts to drop immediately.

So, to make the bullet hit the target, the rifle scope’s plane must cross the rifle barrel’s plane at some point. As the bullet moves through space, it first crosses the scope’s plane close to the rifle. Then, it rises above that plane. Downrange, it crosses the plane again.

This second crossing is where your target is supposed to be.

Trajectory

Simply put, the trajectory is the bullet’s path downrange.

Looking at the chart, you’ll see the bullet crosses the rifle scope’s line of sight somewhere around 30 yards downrange. This is why some people start sighting their rifles in at 30 yards. 

Because they understand the bullet trajectory, they can sight in very quickly at 100, 200, 300, or more yards downrange.

Ballistics

Ballistics matters a lot. Every bullet has different ballistic characteristics. Different bullet weights in the same cartridge will have different ballistics.

Some modern rifle scopes with electronics come with a built-in ballistic chart. The scope automatically adjusts the elevation of the bullet for the distance to the target.

Other scopes come with a Christmas tree reticle. Shooters use this to adjust their point of aim through the scope, to make the bullet hit the target. This, however, requires a good understanding of the bullet’s trajectory.

Adjustments going up or down refer to elevation (or distance). 

Windage refers to moving the reticle left or right, and this adjustment is required because the wind will affect the bullet’s flight path. 

Some advanced electronic scopes can compensate for this if you put in the wind speed. Using a Christmas tree, as pictured above, also requires knowing the wind speed and how much that wind will deflect the bullet.

Rings

That said, the rings that hold your rifle scope to the gun can provide some windage relief. One or both of the mounts has an adjustment bolt to slide it left or right. Scope rings can also affect elevation. These rings are sold as low, medium, high, or very high rings.

If you need large amounts of elevation relief, then you’ll need to get a custom mount for the rings. 

This mount is built so that the front part is a bit shorter than the rear. The scope actually angles down enough to see with the naked eye. These kinds of mounts are used for shooters trying to hit targets at a mile and beyond.

How Does Scope Magnification Work?

Scope magnification works by zooming in on a target to make it appear larger through the scope. The lenses inside adjust the incoming light into a larger or smaller beam of light going through the scope tube. 

Some electronic scopes have digital zoom, and others have both digital and optical magnification. 

Digital simply enlarges the image on the screen. When you do this, the picture becomes distorted or pixelated. Your target might be harder to see against the background in this case.

Variable Power Scope

Some scopes have a fixed magnification lens, meaning you can’t change the magnification. Other scopes may allow you to zoom in or out – these are variable power scopes. Having this feature is important when shooting at a long distance.

If you’re shooting at something small, say a prairie dog, then being able to zoom in lets you better center the crosshairs for the best shot – you can see more of the animal and position the reticle appropriately.

When purchasing a scope, you’ll see a number representing the magnification lens level. A 3x level means your target is three times as big as seen through the scope vs. looking at it with your naked eye. A 10x means the image is enlarged 10 times, and so on.

Rifle Scope Objective Lens Adjustment

Knowing how to use an adjustable objective scope is important if you want your target in focus. Getting a sharp focus is especially crucial when you’re hunting certain animals. 

Many states have antler restrictions on deer; for instance, you must know how many points the deer has before you pull the trigger. Small bucks get to walk away, and trophy bucks get to go home with you. If the image in the scope is fuzzy, you might accidentally shoot an under-sized deer.

The objective lens adjustment is rarely made with rifle scope adjustment knobs. Most of the time, it’s done by adjusting the tube the objective lens sits in. To make adjustments, simply turn the objective lens housing in or out until it’s set to your preferences.

Ocular Lens Adjustment

You adjust the ocular lens, the one on the back and closest to your eye, the same way.

Some high-end scopes will have a focus knob, but either way, you’ll need to set the ocular lens focus manually.

If you wear glasses, you may have to readjust the ocular focus whenever you take your glasses off to look.

Some scopes need to be focused at various distances, and some are fixed. Rifle scopes with a high magnification level often have an adjustable focus. Rimfire scopes, which are used at 100 yards and less, are a focus-and-forget-it setup.

Zeroing

Setting your scope so the reticle and the bullet’s point of impact are in the same spot is called zeroing your rifle. If you remove your scope, you’ll have to re-zero it when you put it back on the gun. Ocular lens focus matters here too.

It’s always a good idea to check zero before hunting season opens and a few times during the season.

Sometimes a rifle scope won’t hold zero. This can be traced to three main problems. The first two will require you to change the mount.

1. Poor mounting – If your scope rings are not tight, the rifle scope will move in the mounts. In that case, it’ll never stay zeroed. You may need to tighten the rings or get new ones.
2. The scope is twisted in the mount – You may not be able to see this with your naked eye.
3. The rifle scope is broken – Cheap scopes and heavy-recoil guns do not go together. The gun will break the erector set inside the scope. If that happens when you’re drawing down on a trophy, well, better luck next time, and invest in a good rifle scope.

In cases 1 and 2, a rifle scope mounting kit will help you secure the scope to the rings and make sure it’s not torqued. I keep one of these kits in one of my gun toolboxes – it’s certainly worth the investment.

Rifle Scope Hardware

Here are the essential parts of the hardware in a scope. Things like the tube length, size, and material used to make the tube and rings have no bearing on how the scope works.

All scopes are focal plane scopes, meaning the crosshairs stay in focus as you focus on the target. The focal point is a matter of personal preference. Some prefer a second focal plane, and others prefer a first focal plane scope.

Lens

All scopes have at least one lens. It’s made from shaped glass. A scope works by capturing an image in that glass and presenting it to your eye.

The glass inside is ground and polished to make a lens. This lens lets you focus on the reticle inside the scope and the target at the same time, which is a distinct advantage over open or iron sights. When you use iron sights, you must choose to focus on the target, the front sight, or the rear sight.

As the distance between the target and the shooter increases, the ability to focus on the scope reticle and the target at the same time becomes more and more important. Frankly, you don’t need a scope to shoot accurately at long range; a scope just makes this kind of shooting much easier.

Glass Is Critical

The glass is a critical component of any scope. The quality of the glass directly affects how clear your view through the scope is. Poorly made lenses will distort images, especially as the distance grows. 

Top manufacturers coat their lenses to reduce glare. Think about sunlight reflecting off glass or other vehicles as you drive. That glare affects how much you can see. 

The same applies to scopes. Less glare means a better view.

Color Shift

Long-range scope lenses are also coated to correct a red-blue-green color shift. 

The human eye only sees red, blue, and green as colors. Each of these colors will have a slightly different focus distance. The human vision system handles this until the eye looks through a scope.

Ideally, all three colors should focus on the same plane – the retina in your eye, in this case. At long distances magnified through a scope, the focus planes are different enough to be noticeable unless corrected. An uncorrected image will appear fuzzy.

A lens coating corrects this and focuses all three colors onto the same plane, giving you a clear image of the target.

Glass is still critical in red dot and reflex scopes, even though these optics typically don’t offer magnification. Poorly made lenses will distort the reticle, the target, or both. 

If the target is not clear, no matter the reason, shooting accurately will be difficult.

Types of Scopes

Scopes can actually be separated into two different groups: either traditional or powered.

Traditional

A traditional scope is a tube with glass lenses inside. It does not need any outside power because it uses ambient light. 

However, this then creates the drawback to using traditional scopes – you need a light source.

In the twilight, finding the target can be hard with this kind of scope. Picking out a deer moving along the tree line at 250 yards when the sun is below the horizon is very hard to do.

Night hunters using traditional scopes need a spotlight. This means packing in a light and a power source, all of which add weight to the needed gear. Big spotlights require a second person to man the light while the first person shoots. Not to mention, spotlights can often spook animals, which is another drawback.

The advantage of a traditional scope is it doesn’t need batteries. Mount the scope, sight it in, and you’re ready to shoot. You never have to worry about batteries dying or being left in the scope so long the acid leaches out and destroys the scope.

Some traditional scopes have a lighted reticle that does require a battery, but these scopes can be used without lighting the reticle.

Powered

Powered scopes need electricity from a battery to work. Interestingly enough, a few companies are making red dot and reflex scopes with solar panels, using the panels to charge a battery inside the scope. 

Without power, you’ll just be looking through a pane of glass with no reticle or looking at a dead screen.

Night vision, thermal, red dot, and reflex scopes are powered. A traditional scope with a lighted reticle is not considered a powered scope.

Powered scopes work in two ways:

1. The reticle is generated by the electronics. Red dot and reflex scopes have this setup. 
2. Night vision and thermal scopes generate an image and present this on a screen along with the reticle and other information.

The obvious drawback here is the need for power. Without that flow of electricity, the scope is useless and is just extra weight. If the batteries die mid-hunt and you don’t have replacements, the hunt is over.

However, note that battery life ranges from just a few hours in thermal and night vision optics to hundreds of hours in red dot and reflex scopes.

What Makes Rifle Scopes Different?

What makes rifle scopes different from telescopes and binoculars is the rifle scope is meant to be mounted to a weapon. While it can be removed, mounting is usually called permanent because you don’t take it off between shots.

Rifle scopes also have reticles. Telescopes do not. Rangefinder monoculars will have a chart visible through the device to show the distance to target, but this isn’t considered a reticle. 

Some advanced rifle scopes can compute the distance to the target. Binoculars may have some sort of built-in rangefinder, but this is not the same as a rifle reticle.

Additionally, rifle scopes also have a single tube. Binoculars have 2 tubes that focus light independently down each tube. If you wear glasses, you’ll need to adjust the focus on each tube to your vision when you don’t wear glasses while using binoculars.

Frequently Asked Questions

How does a sniper scope work?

A sniper scope works exactly the same as any other scope. It’s just designed for long-range shooting. It also works perfectly for hunting.

How does a rifle scope work internally?

How a rifle scope works internally is by focusing the light reflected off your target. The zoom feature makes that image appear larger than what you see with your naked eye. Note that fixed power scopes will not zoom.

What are the 3 dials on a scope for?

The three dials on a scope are for:

1. Elevation, on the top.
2. Windage, on the right.
3. Parallax or focus on the left.

How does the power work on a scope?

The power on a scope works by magnifying the image you see through the scope. A fixed power scope cannot be changed. Variable power scopes allow you to zoom in or out.

For similar reading see: which type of sight is most accurate and gives the best view of the target.