Scopes 101: Understand How Scopes Work


Once again I am disappointed by the quality of the articles on the internet, but this time around rifle scopes. On my search, I looked for a quick tutorial that would not waste my user’s time, while providing the relevant information and photos. Since I could not find anything, I have made a quick, simple and clean guide on understanding riflescopes.

Quick Math Lesson

Time to find out that you really will use what you learned in trigonometry class in the real world; heck if they told us that it was needed for guns and shooting I know I would have paid more attention in the first place.

First, let’s get the basic’s out of the way. U.S. manufacturers have settled on 100 yards to be the standard distance as the basis for range calculations. Europeans on the other hand primarily use 100 meters as their set variable. Most all other countries also utilize the metric system (meters) so make sure to verify this statistic when selecting or sighting in a scope.

Note: 100 yards is not 100 meters and 1 MOA is not 1 Mrad

Scope Measurements (Fixed vs. Variable):

Variable power scopes are typically described by 3 numbers, i.e. 3-9×40. While fixed power scopes are described by two numbers, i.e. 10X40. These number pairs are the top-level specs for the scope. I will explain how all of this works on a variable power scope first then show how a fixed power is represented afterward.


The first 2 numbers specify the magnification levels that the scope from the lowest level to highest level. In our example, the first number is 3, which means that on the lowest magnification setting the scope with magnifying the image viewed through it 3 times what you can see with the naked eye. The second number after the dash is 9 signifying that the highest magnification that the scope is capable of is 9 times magnification of the image. The larger number after the x in the scope designation is the diameter of the objective lens (big lenses in the front of the scope) almost always notated in millimeters. So to finish the example a 3-9×40 scope is a variable power scope that has a magnification range from 3x to 9x with a 40mm diameter objective lens.


Now that you understand that, you can apply the same denotation to fixed power scopes, but since they only have one level of magnification there is only one number before the objective lens measurement. So our example fixed power scope of 10×40 has a fixed magnification of 10x and an objective lens diameter of 40mm.

Field of View:

Field of View (FOV) is simply how wide of an image you can see through the scope. More specifically the measurement is listed as the amount of area you can see from left to right at 100 yards away with a specific magnification level. In more technical terms, it is the amount of feet per 100 yards. As you increase your magnification, your FOV will decrease.


3x Magnification = 30 feet (FOV)

9x Magnification = 14 feet (FOV)

Light Transmission:

I started to write about this topic, but realized is does not really matter! All you need to understand is that the best scopes on the market only use 94% or less of the usable light in the area. Most manufacturers no longer tell you what the usable light percent is any longer. So don’t worry about it. I might decide to write a full article on this later.

Ocular (Eyepiece) and Objective Lenses:

Ocular Lens

The ocular lens is the lens at the rear of the scope that helps focus the light into an exit pupil where your eye will see the image. The most important factor of the ocular lens is the exit pupil diameter it creates. The exit pupil diameter is determined by the amount of magnification you set on your scope. As the magnification increases, the exit pupil will get smaller.

To break it down to simple terms, your exit pupil should match that of your own pupil diameter. For those that don’t know, your pupil expands and contracts depending on the amount of light in the area. The brighter the light, the smaller your pupil will be and vice versa. The human eye on average has a pupil diameter for about 1.5 to 8mm depending on lighting conditions.

For Example, A high power variable scope that does not have a large exit pupil will cause you issues in low and high light situations as your pupil adjusts because the pupil on the high power scope will give you less opportunity to adjust the exit pupil size.

Objective Lens

The objective lens is on the front of the scope and determines the amount of light that enters the scope. Light is important as you need enough so that the optics inside the scope can develop a clear picture. This makes the number one factor of the objective lens, the diameter. That being said, there are a few downsides. First, a larger objective lens requires that you mount your scope higher and, therefore, throw off your ability to properly sight and comfortably view objects through the scope without adjusting your head position. The second downside is that they are typically heavily.

Lens Quality and Coatings

Additional factors to consider for the objective lens is the type of glass used to make the lens. Higher quality scopes use high-quality low dispersion glass that helps keep the light in a nice tight beam and prevents it from spreading out like a prism. This enables the scope to produce crisp clear images that can make all the difference when you are trying to identify a target that is against a similar background.

In most quality optics the objective lens is multi-coated to filter the wavelengths of light that the human eye can see best and protect against damage. In most cases you should be good with an objective lens no larger than 42mm. General use scopes are usually 28-30 mm diameter and depending on the quality of components will serve you well.

Windage and Elevation Adjustment:

These two adjustments allow the shooter to align the crosshairs with the bullet trajectory. Depending on the model and intended usage of the scope, these adjustments can be made in different ways. Most standard scopes and scopes with reticles that compensate for range and bullet drop will have adjustment knobs that require a tool to adjust and covers to protect them. The intended action is that the scope will be zeroed at a set range and load; the shooter can then use the reticle to make an adjustment to their point of aim.

For shooters that would prefer to adjust the point of aim of the scope to account for range and field conditions, some scopes come with what are known as target turrets. These are windage and elevation knobs that are usually not protected with caps and are larger for shooters to turn with their hands. Like regular windage and elevation knobs shooters usually zero the scope at a known distance and load and adjust the markings on the turret to zero so that they can be adjusted back to a known location.

Windage Knob (Side) – Adjust the point of aim horizontally (left or right)

Elevation Knob (Top) – Adjust the point of aim vertically (up or down)

When adjusting scopes, it is regularly practiced that you only make adjustments in one direction on each knob. This ensures that any small amount of slack in the adjustment system is always accounted for in the same way on the scope. In order to do these elevation adjustments are usually always made in the up direction, so when zeroing your scope, you start with it low and slowly work your way up. If you go too far up you can always go down by first over adjusting down then coming back up to your planned elevation.

For windage direction, it is best to pick a side and always move that way. I always choose to adjust right so I start far left and adjust right until I am zeroed. If I need to adjust left I first over adjust left then adjust right until I reach my desired windage. This takes a little practice at first but once you have it down it saves a lot of frustration trying to follow shots around a target.

Scope Adjustment Units

Scopes are adjusted in angular units that can be equated to linear distances at known ranges. There are two main units of measure that are used, Minute of Arc (MOA) and Milliradian (Mrad). I will not argue that either is better but that most U.S. manufactured scopes will use MOA and most European manufactured and military scopes will use Mrad.

Most manufacturers adjust MOA to be ¼ MOA* per adjustment click. This equates to roughly “¼” of linear movement per adjustment click at 100 yards range. When using the Mrad most manufacturers index each adjustment to 1/10 Mrad per adjustment click which equates to about 1cm at 100 meters.

*MOA- 1.04 inches at 100 yards, in this case, it is ¼ of that measurement at 100 yards per click

Focus Adjustment:

Many scopes have a focus adjustment control either as a knob or ring that can be turned. The focus adjustment lets the shooter bring the target into focus at varying distances to reduce the effect of parallax or the moving of the point of aim depending on the movement of the shooter’s eye. Shooters run into this problem most commonly when they are using a scope at close distances since the optics are usually focused for targets past 80 yards. To counter this, many manufactures allow you to focus the scope manually.


The reticle is the adjustable aiming crosshair usually located in the center of the scope. There are more reticle patterns available for scopes than I am willing to write about so we will discuss a few different types and let you decide on your favorite pattern. To break things down and make things simple you have standard crosshairs (2 lines that intersect), Calculator reticle (measurement lines to calculate distance, length or holdovers) and Compensation reticles (multiple crosshairs to compensate for specific distances or windage). Of these many types of a reticle, many of them can have extra illumination from natural light, electronic light, chemical light, or a mixture of any of these. Whew that was a lot, let’s break it all down.

Plain reticles are pretty easy to use and provide a clean image and are the least popular. Calculator reticles are one of the more popular reticle types after compensated reticles. Calculator reticles usually contain shapes or grids of a known MOA or Mrad measurement to aid in the evaluation of a target. These are the reticles that target shooters use to evaluate their range from a target based off of the known size of an object that is at the same range as the target. Users of these reticles usually use easily adjustable windage and elevation knobs to adjust their point of aim based on calculations from the reticle. Some calculator reticles will also contain a grid that allows the shooter to hold over their target after making measurements.

Compensation reticles are the most popular as they take the place of the calculator reticle to inform the shooter of a pre-calculated holdover for a specific range and load combination. This reticle makes it very easy for a shooter to take shots without having to make any adjustments or perform any calculations. There are a few drawbacks to this reticle, though. In order to use this reticle effectively, it is necessary to know your range from the target and an accurate compensation reticle is specific to an individual barrel load combination. It is necessary to know what load combination the reticle is calibrated to use.

Tube Diameter:

The standard scope tube diameter is 1 inch but recently more scopes are being produced with a 30mm tube. While there is not a problem with the 1-inch tube the 30mm tube does offer a few advantages. The 30mm tube allows the manufacturer to use larger diameter lenses throughout the entire scope. The larger size allows room for more adjustment range of the windage and elevation adjustments. Also, the larger diameter tube provides more surface area to contact the mounts resulting in a stronger mount that is less susceptible to being moved by impact or recoil. Now that 30mm scopes have grown in popularity there seems to be a good selection of mounts and rings to fit most firearms.

Eye Relief

Eye relief is the distance from the rear of the scope to your eye where the exit pupil is in focus. Most rifle scopes will have a standard eye relief of about 4 inches, but you can find different models with varying levels of eye relief. The standard eye relief is usually acceptable for most rifles and mounting situations to provide enough room for a proper cheek weld and room for the recoil. It is a lot better to absorb the recoil through your shoulder than through your eye socket. Some scopes designed for very heavy recoiling rifles will have an increased eye relief to help prevent this.

Mounting and Sighting:

When mounting a scope, it is ideal to try to have the centerline of the optics as close to the centerline of the bore as possible taking into account the size of the scope. To find the best placement for the scope lift the gun as if you were in the field firing it. DO NOT, I repeat, DO NOT mount a scope based on a sitting position at a gun range. You want the scope to be in a natural position as if you were in the field shooting. So worry about where your face sits comfortably and then place the scope to sight it in. After that, it is important to select a mount that will securely hold the scope to the rifle and keep everything aligned.

There are two main types of mounts, one piece or two piece rings. One piece mounts have the advantage that the rings are usually already aligned and they are easier to mount. Two piece rings need to me aligned correctly usually with a tool. Some gunsmiths will lap the rings to ensure that they are perfectly aligned with no burs in order to get the best fit possible with the scope.

Once the rings or base is mounted to the rifle the scope can be mounted level using the manufactures recommended torque settings. When working with precise alignments and scopes, it is important to pay attention to these torque values as overtightening can damage your scope, mount or rifle or all three. Once the scope is mounted it is easy to use a bore sight to get the scope close so when you take it to the range to zero you will, at least, be on paper to start.

*Note: Mounts for scopes come in different heights as well. Determining the proper height can be difficult, but using a calculator like this one can help make sure that your scope and rings will fit on your rifle. (


As someone once asked me, how much money do you have? You can spend as much or as little on a scope as you want. There are many great scopes on the market, and many of them cost a pretty penny. We recommend that you first decided what you want to do with your scope and what features you want it to have. Armed with that list, you should be able to find either a new or gently used scope for a fair price. Do your research and check out the site down the road, as we will be reviewing quite a few scopes ourselves.

TheARGuy’s Opinion:

A good rule of thumb when purchasing a scope is to pay more for it than you did for the gun you are mounting it on. A good scope will make or break your accuracy at a range or in the field. A quality scope will last a lifetime and usually have a warranty to back that. We recommend choosing a scope from a quality manufacture that is open about the specs of the materials used and has a good reputation for quality. Just like rifles you can find scopes that are engineered for specific task and ones that can handle a range of tasks. Good glass is worth the investment and you will be happy to have it out in the field or at the range.

6 Responses

  1. Harold says:

    What is a “exit pupil”? TIA

    • Aaron Brown says:


      This might help. It talks about ocular lens first, but ultimately explains the exit pupil.

      The ocular lens also commonly referred to as the eyepiece will typically show a small circle of white light if you hold the scope out. This white light is referred to as the exit pupil. The exit pupil explains the statement I made above about light vs. magnification. If you turn up the magnification, you will notice the exit pupil gets smaller because it’s blocking out more light. As for the ocular lens, there is not much to say out it. You will typically have a power nob that adjusts the power between the scopes range. (i.e. 3-9x)

  2. Vincent says:

    If you judge scopes by price alone as your rule of thumb. You are an idiot. If you think the price of your scope will make your break your accuracy your wrong. A good shooter with a steady hand can shoot any functional scope accurately.

    • abraun says:

      Vincent, we completely agree. I wouldn’t justify calling anyone an idiot, but purchasing a scope based on price is never a good option. Research and experience are the two largest factors in selecting a scope. As for the article the comment “A good rule of thumb when purchasing a scope is to pay more for it than you did for the gun you are mounting it on” is to prepare newcomers that scopes are not cheap, it does not mean to purchase a scope based on price. In most cases, a quality scope will cost as much if not more then the gun it is mounted on. That being said, there are some great low-cost options that have been introduced into the market by both Nikon and Bushnell over the past couple of years. I started with a Nikon and still use it on my hunting guns to this day and it costs less than my gun. The only recommendation to our readers is to avoid Chinese knock-offs. Unfortunately, they are popping up all over the web and even at gun shows. The vendors that sell these products should be banned from these shows, but they pay for space so it won’t change anytime soon.

  3. Aaron says:

    Nice write-up, abraun. You’re right, it’s difficult to find a succinct and simple write-up about optics so thanks for your work on this.

  4. george mouzakis says:

    Abraun, thanks for the work. Sometime perhaps you could screen capture some reticle images and use balloons to point out the features/nomenclature

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