In this article I will take a more detailed look at a modern, variable powered riflescope and what goes on inside as you start turning knobs and power rings.The illustrations depicted in this article will be pretty basic though because the actual design and location of some of the internal components will vary from manufacturer to manufacturer. Despite this I hope that at the end of this article you will have a better understanding of how a variable powered riflescope functions and better appreciate the advantages this piece of technology can offer.
First lets go through the basic nomenclature of a modern variable powered riflescope both inside and out.
On the Outside:
On The Inside:
What Do The Numbers Mean?
The meaning to all of the numbers gets a lot of people confused and in reality the answer is pretty simple. If you receive a scope that has 3-12X50, those numbers tell the user several things. The first two numbers represent the magnification range with three power being the low end and twelve power being the high end. The next number is the objective lens diameter, which in this case is 50mm. It's important to note that this isn't the outer diameter of the objective bell; it is only the diameter of the objective lens. It is important to understand the meaning of these numbers since it can be handy information when it comes time to select a scope or choose the proper mounting solutions. Failing to understand it can lead to purchasing the wrong scope for the job or rings that are the incorrect height.
The Main Tube
The main tube for a riflescope can either be a one or two-piece design, typically made out of high quality aluminum, however steel and titanium have been used as well. The main tube primarily contains and protects the major components of the riflescope in addition to providing a surface area for the scope rings to attach to. The finish most often used for the main tube and other external components is a Type II or Type III hard anondizing that can be just about any color specified by the manufacturer, however black is most common.
Reticles come in all manner of shapes and sizes so that the shooter can choose the one that will best fit his or her needs and situation. Some are simple crosshairs while others are more complicated with additional dots and/or hash marks that can be used for a variety of purposes. The distance between the dots and/or hashes will also depend on what system the reticle was designed around, being either milliradian or minute of angle-based. There are a few examples of both kinds shown below from a sampling of manufacturers that make quality riflescopes.
It is interesting to note that early riflescopes had simple crosshair-type reticles that were made of spider web. This was used because at the time the spider web was many times stronger than the wire available at the time, so it could withstand the recoil from heavy recoiling rifles. As time went on, stronger types of wire started to replace spider web as the material of choice when making scope reticles. While wire reticles are still used in some riflescopes, reticles etched in glass are common in many of the riflescopes used by military, law enforcement, and civilian shooters. Etched glass reticles offer a wider array of reticle styles and illumination options that were once just not possible. The reticle is on a flat piece of glass that is installed and epoxied into a reticle cell, which is in turn attached to the erector later on.
Adjusting the Diopter / Focusing The Reticle
Properly adjusting the diopter on a riflescope is something that many shooters, particularly new shooters, misunderstand the importance of. I've seen and heard many shooters claim that their scope was broken because they couldn't get the target image in focus by turning the ocular. In reality though, their scope is probably fine, it's just that they don't fully understand how the scope works. By turning the ocular, or ocular assembly, to the right or left, the shooter is increasing or decreasing the distance from ocular to the reticle, in turn focusing it to their eye, nothing more. Some people also mistakenly turn the ocular all the way in when adjusting the diopter and calling it good because the reticle looks sharp, when it's really not optimal. The user should only adjust the diopter to the point the reticle is nice and crisp against a light background, even when turning away and looking back through the scope. Also keep in mind that the diopter setting will be different from person to person since no two eyes, even in the same skull, are the same.
First Focal Plane and Second Focal Plane Reticles
Your typical, modern-day variable powered riflescope will have the reticle installed in one of two ways; it will either be located in the first or second focal plane. For most shooters the choice between first and second focal plane is based on cost, how they will be using the optics, and pure personal preference. In general though a scope with a first focal plane reticle will cost more than a one with a second focal plane reticle, even if they are the same magnification level. The reason for this is that designing and producing a scope with a first focal plane reticle is far more expensive and time consuming.
I'd like to first talk about reticles that are located in the first focal plane and what that means for the shooter. It wasn't but a few years ago that most of the scopes sold in the US that had first focal plane reticles came from European manufacturers.However, that is beginning to change with more and more domestic companies making the switch from SFP to FFP on some of their models. This is starting to bring the cost down on some models and open up the possibilities for many shooters to have an optic that is extremely versatile.
When I talk about first focal plane and second focal plane what I'm really getting at is the location of the reticle cell on the erector and inside the main tube. In first focal plane optics the reticle cell is located at the front of the erector and forward of the lenses in the erector that help control the magnification level. This means that as the magnification level changes the reticle will appear to get bigger and smaller from the shooter's perspective. This is where many shooters misunderstand and even start to dislike FFP reticles because they say that the reticle covers too much on high power and gets too small on low power. Now while the reticle does appear to be change size it is in fact maintaining its size in relation to the target. This means that if the target is five mils tall on high power then it will still be five mils tall on low power. Since the sub tension of the reticle doesn't change based on the magnification, all of your range estimation, trajectory compensation, and leads can be done on whichever magnification level is desired. This gives the tactical shooter a big advantage in the field, in competition, or when their life is on the line.
Probably the most common arrangement for a variable powered riflescope is to have the reticle installed in the second focal plane. This arrangement can be found from nearly every domestic optics company in nearly every price bracket. This set up is relatively cheap to design and easy to produce when compared to optics with a first focal plane reticle. Unlike the first focal plane reticle the reticle cell is installed at the end of the erector tube, close to the magnification ring. This places it behind the lenses that control the magnification level meaning that as the magnification level changes, the reticle will appear to stay the same size from the shooter's perspective. In reality though, as the target image gets smaller the reticle is covering up more of the target since it isn't changing in relation to the target. What this also means is that normally accurate ranging, hold-overs, and leads can only be done on one magnification setting without some sort of conversion.
Both FFP and SFP arrangements have their advantages and disadvantages it is up to the end user's preferences, needs, and checkbook to decide which is best.
The Erector Assembly
One of the more complicated components of the riflescope it has numerous parts that are vital to the function and operation of the riflescope. This tube shaped assembly houses the reticle cell and a group of lenses that help control the magnification level of the scope. If the scope is equipped with an illuminated reticle, often times small LED lights and wires will be attached to the erector as well so it can be linked to the illumination knob. There is also the erector spring, which pushes against the erector to apply force against the two contact points for the elevation and windage to ensure consistent travel. Not only that but the erector also has to pivot smoothly at a point near the end of the erector so that it can actually move when pushed by the spring and or the elevation and windage knobs. Now that is a lot of stuff to be going on inside of the scope and hopefully this section will shed some light on exactly how it works.
When the user grasps the magnification ring and turns it to select a higher or lower power, not much thought is given to what goes inside to make that all happen. As the user is turning the magnification ring, it is connected to a portion of the erector that turns along with it and moves a pair of lenses inside of a scroll. The position of these lenses inside the erector determines how magnified the target will be from the shooter's perspective within the magnification range of the scope. As the lenses come closer together towards the front of the erector tube, the more magnified the target is to the shooter.
The more spread out and farther away from the objective lens they become, the lower the magnification level.
When the user turns the elevation or windage knob there are several parts that work together in order to make the bullet reach a certain distance and hit a specific target. Generally almost all tactical scopes have an elevation and windage assembly that the knobs slip over and attaches to. These assemblies contain numerous parts but mainly they house the click element and a spindle, typically made out of the brass, which protrudes down into the scope.
The brass spindle will be tapped for a certain thread pitch that is based on the manufacturer's desired click value for the scope. This means that the thread pitch for a scope with ¼ MOA adjustments will quite different than one with 0.1 MRAD adjustments. Now, the click element that is also contained inside of the assembly helps to ensure that the spindle only moves an incremental amount to correspond with the desired click value. The most common click values in the tactical scope market are the aforementioned 1/4 MOA and 0.1 MRAD click values. Now what do those two values mean? Well, for every one click of the knob on a scope with 1/4 MOA click values, the point of aim moves 1/4" at 100 yards, or .26" for the True MOA crowd. The same pretty goes for 0.1 MRAD click values as well, one click of the knob moves the point of aim .36" at 100 yards, or .1 milliradian. The milliradian click values have been gaining immense popularity since no conversion is really necessary to go from what you see in the reticle to what you have to dial in for a correction. Now, if either part of the click element or brass spindle is assembled incorrectly at the factory, the scopes adjustments will not work out to what they are supposed to.
Now moving farther down to the erector….if the user is looking through the riflescope while turning the elevation or windage knob, they may notice that the reticle appears to move in the opposite direction from what they think it should. The reason for this is that the image that reaches the erector is actually upside down. When the user dials in more elevation, the erector is pushed upwards by the erector spring, pivoting at the rear of the erector on a sort of gimbal or ball-and-socket joint, making it appear to go down on the target image.
The same goes for when less elevation is dialed into the scope, the reticle looks like it is going up when in fact it is being pushed down by the elevation spindle.
Looking from the ocular through the erector:
The erector spring really makes all of this possible since it has to be able to push the erector up into the spindles to ensure everything tracks as it is supposed to throughout the advertised range of adjustment. If it is too weak it will not be able to push it all the way up or to the side, resulting in the user experiencing “ghost clicks” towards the upper end of the travel range. This means that the user can turn the elevation or windage knob but no change in impact will occur, which can mean bad news for long range shooters. Sometimes a poor quality spring is to blame for this issue and sometimes it's a result of being overly compressed for an extended period of time. Something that I've always found interesting was that only a few millimeters of travel was necessary to reach the extreme limits of the scopes elevation and windage travel. For example if you were shooting to about 1000 yards and dialed in the elevation for the shot, the erector inside of the scope may have only moved about a millimeter depending on the cartridge that's being used.
Adjusting For Parallax
Parallax, to some degree, is present in almost all magnified optics and comes from the fact that the target image and the reticle are not focused onto the same focal plane. In optics with a lower magnification this does not present much of a problem but as you go higher in magnification some sort of compensation is required.
There are three ways to deal with parallax or compensate for it, have a scope with a fixed parallax setting, have an adjustable objective, or have a side focus knob. Fixed parallax scopes fix the focus lens in place and adjust the position of the objective lens in or out until the image is parallax free at a certain distance. Adjustable objectives also have the focus lens fixed in place but the objective assembly moves in and out by means of an outer ring on the objective bell. Scopes with a side parallax knobs deal with parallax in a different way by moving the focus lens instead of the objective. The focus lens is attached to the parallax knob by means of an arm that's in turn screwed to a slider riding inside of the parallax knob. As the parallax knob is turned, the slider moves inside of a scroll, pushing or pulling the focus lens, and allowing the image to become parallax free. Typically tactical riflescopes are able to compensate for parallax from 50 yards, or meters, to infinity, although some can go down as low as 10 meters.
Another interesting fact is that for most tactical scopes with side focus knobs, getting focused from 50 yards to infinity only requires moving the focus lens about a quarter of an inch.
In The End
What I've just described above was how your basic tactical riflescope works on the inside, as you would normally use it to make corrections and focus the target. Keep in mind that there can be ten or more actual lenses in the scope and upwards of a couple hundred individual parts depending on the brand and features. So you can see why some of the illustrations were pretty basic since it gets a little more complicated though when you start talking about zero-stops, illumination knobs, and other features that come on many high scopes out on the market. Hopefully though, this article has given you a better understanding of what is going inside your average tactical riflescope.