Call:860-808-1281
In its simplest form, the rifle scope is nothing more than a tube that houses lenses for magnification and a reticle for indicating your bullet’s point of impact, essentially a small telescope with cross hairs. The Main tube can be of either a one or two piece construction. Tubes are most often constructed of high quality aluminum, commonly noted as “aircraft aluminum”, but have also been built from titanium and steel. Aluminum is probably your best bet, titanium can be heavier and more costly, while steel requires more attention to prevent rust.
External components of a rifle scope
The end of the scope furthest from your eye is called the objective bell, which coincidentally houses the objective lens. This is the light gathering apparatus, and the diameter of the objective will determine how much light is allowed into the scope to be transmitted to your eye. I remember this term because the objective is closer to the object of my attention. The other end of the scope is called the ocular, or eye piece.
In addition to housing and protecting the internal components of the scope, the main tube also provides the surface area used to mount the scope, and holds the adjustment controls used to fine tune your scope’s performance. There is some good news here, in that some of the terminology will be familiar to anyone who has worked with open sights before. Located at about the midpoint of the tube are knobs to adjust for windage and elevation. The windage adjustment moves the reticle to correct your bullet’s point of impact in the right to left plane, and is most often located on the right side of the tube. The elevation adjustment corrects point of impact in the up and down direction. Adjustments are made by “clicks”. There are two main click values, 0.1 miliradians (MRAD), and ¼ Minute of Angle (MOA). The 0.1 MRAD clicks change point of impact by 0.26 inches at 100 yards while the ¼ MOA clicks change point of impact by ¼ inch at 100 yards. The mechanism consists of a spindle, a spring, and a click element, all calibrated to ensure that adjustments occur only incrementally and correspond to desired values.
Some scopes have a third knob which adjusts for parallax (don’t worry, we’ll get to that in a bit). Scopes that lack a parallax knob may have an adjustable objective for this purpose, this is controlled by rotating the objective bell. These scopes are called adjustable objective or AO scopes. Back at the ocular end there may be an additional adjustment for the diopter, which focusses the reticle, again by rotating the ocular. A frequent mistake made by folks who don’t fully understand the functioning of their scope is to believe that they can focus in on their target by adjusting diopter at the ocular. In reality, this adjustment is only used to achieve proper focus of the reticle, not the target.
The final external adjustment is the power ring. This is a feature of variable power scopes and it serves to change the power, or magnification, of your scope. It is located towards the rear of the tube, between the ocular and the adjustment knobs and is operated by rotating the ring to the desired magnification setting for the shooting situation of the moment.
Internal Anatomy of a Rifle Scope
The anatomy gets a little more complex on the inside of the scope, but we can take it! The heart and soul of your scope are the lenses. From front to back you have an objective lens, a focus lens, magnification lenses, and ocular lens. In the majority of modern rifle scopes, the lenses are coated to reduce glare or reflection. The less light a lens reflects the more light is transmitted back towards the eye. Coated lenses can make a big difference in the light gathering characteristics of a scope, increasing the utility in low light shooting conditions. The objective lens is the primary light gathering lens, so the diameter and the coating on this lens can make or break your scope in terms of low light abilities. The objective lens (or lenses) is housed inside the tube in the objective lens assembly. If your scope is an AO model, parallax can be adjusted for by rotating the adjustable objective and changing the position of the objective lens.
The next lens back is the focus lens, housed in the focus lens assembly. This lens can be fixed or adjustable. In a fixed focus scope, the focus lens is factory set to be parallax-free at a specific range, commonly 100 yards or 100 meters. In models with a parallax adjustment knob, rotating the knob adjusts for parallax (we’ll get to that!) at varying ranges by moving the assembly closer and further from the objective lens. In practice this means that as you adjust the knob towards infinity the lens gets closer to the objective lens. When adjusting for closer objects the assembly moves back towards the ocular. Generally, scopes with parallax adjustments can be dialed in to be parallax free at ranges varying from 50 yards to infinity.
Continuing backwards towards the ocular within the main tube we next arrive at the erector tube. The erector tube houses both the magnifying lenses and the reticle assembly. In a variable power scope the lenses change position within the erector tube as the power ring is rotated, closer to the objective on high power and closer to the ocular on low power.
The reticle is what is commonly called the “crosshairs”. The reticle cell can be either in front of or behind the magnifying lenses. The first case, with reticle in front of the lenses, is known as a first focal plane reticle. In this arrangement, changing the magnification of the scope will result in the reticle appearing to change size from the perspective of the shooter. In reality, the reticle is staying the same size in relation to the target. This is a huge check mark on the plus side for the first focal plane reticle. Since the reticle maintains its relative size, a target that is 5 dots tall at low power is still 5 dots tall at high power meaning that all calculations of range, holdover, and elevation stay the same at any power setting. In a second focal plane reticle, the reticle is behind the magnifying lenses. In this configuration the reticle maintains its absolute size from the shooters perspective, but becomes smaller or larger in relation to the target as the magnification is changed. The downside to this is that any ballistics or ranging calculations are only good at a single power setting and must be converted with each change in magnification.
There are a wide variety of reticles available. Some are a simple crosshairs, some are of a three post arrangement. Some reticles have a calibrated system of dots or hash marks along the hairs to aid in on the fly ranging, holdover and windage adjustment estimations. These are called MIL-DOT reticles. There are both lighted and unlighted reticles, and those in the lighted category may be red or green. The reticle that is right for you will depend on how you intend to use your rifle and the lighting conditions you expect to encounter; there is also a healthy dose of personal preference that goes into the decision.
Types of reticles
The erector tube also serves as the interface between the reticle and the adjustment knobs. It is here that the spindles and springs that lead back to the knobs make contact, and it is the erector tube that moves when you make your adjustments. Looking through the scope while adjusting the windage or elevation can be surprising to people. The reticle seems to move in the wrong direction. This happens because the image that arrives at the erector is actually upside down. When less elevation is dialed in the front of the erector tube is pushed down (compressing the erector spring) while the back pivots upwards, the reticle appears higher on the target. The barrel must be tilted downward to reacquire the target; this is where the reduction in elevation comes from. The exact opposite occurs when more elevation is dialed in. Windage works in a similar way, only in the horizontal rather than vertical plane. It is important to have a properly functioning erector spring to get the full range of adjustment. A cheap or worn spring can limit function at the upper end of travel.
The last stop on the tour of the scope is the ocular assembly. This is the eyepiece of your scope. This is also where the diopter is adjusted to bring the reticle into focus, by adjusting the position of the ocular lens, for individual users. It is important to remember not to over adjust the ocular. The only goal you are trying to achieve is to get that reticle sharp and clear, do not try to sharpen or focus the target image with this adjustment!
Some Optical Properties of Note to Shooters
I am going to step back a little from the nomenclature and anatomy, and talk a little about some optical properties that are important in rifle scopes. It gets a little bit jargon heavy at this point, and the similarities between rifle scopes and algebra become very apparent.
The first property of note is power. The power of a scope is simply how much an image is magnified. In a 4X scope, the image you see in the scope appears four times larger than it would to the naked eye. In variable power scopes this is expressed in a range, for example a 4-12X would adjust from an image four times larger than normal to one twelve times larger. Scopes are often designated by power and objective lens diameter, therefore a scope designated 3-12X50 would have a magnification range of 3-12 power and an objective lens diameter of 50mm. Objective lens diameter will become more important to you here in a minute, so keep it in mind!
The next property to look at is the exit pupil diameter. This is not a measured parameter, but a calculated one (algebra!). Exit pupil diameter describes the size of the beam of light that comes out of the ocular and reaches your eye. Exit pupil diameter is calculated by dividing the objective lens diameter by the power of magnification. So, a scope with a 50mm objective lens and a power of 5X would have an exit pupil diameter of 10mm. Where x is the exit pupil diameter, let x=o/p where o is…. sorry, wrong class! Back on task! The exit pupil becomes very important in low light settings, and for dawn and dusk shooting you should look for a scope with an exit pupil diameter of not less than 5mm. 5mm roughly approximates the diameter of your own pupil in low light settings, so you really want to have a scope that transmits an image of at least that size to make full use of the available light. This means that a large objective lens diameter is a good deal in low light conditions.
Eye relief is the distance that the eye has to be from the ocular in order to see the whole exit pupil. The most important thing to remember when considering eye relief in your choice of a scope is your rifles recoil. If you are shooting a high powered rifle with a lot of punch a short eye relief scope could put your peeper in the danger zone for a quick jab from your scopes ocular. Make sure that you select a scope with enough eye relief to keep your face out of harm’s way. Also good to know is the fact that eye relief decreases with increasing magnification. At higher powers your eye has to be closer to the scope to catch the whole range of the exit pupil. Refer to the algebra in the previous paragraph.
The field of view is, in a nut shell, the width of the area covered by the site picture in your scope. It is generally expressed in terms of field of view at a particular range, for example FOV 40’ at 100 yards indicates that at 100 yards your scope picture is 40 feet across. This value is expressed as a range in variable power scopes, and the field of view gets smaller as the magnification gets larger. A large Field of view is helpful when tracking a moving target, the target is less likely to travel out of your field of view and have to be reacquired.
Now, finally, we get to parallax and parallax error. Parallax is an artifact of the fact that the reticle and the target are not focused on the same optical plane. Parallax error refers to a situation where the reticle seems to change position on target when the eye is moved, this can obviously squirrel-up your accuracy. Some scopes have a fixed focal point, and are designed to be parallax free at a certain range, commonly this range is 100 yards or 100m. Other scopes have adjustments to compensate for parallax, this can be done either with an adjustable objective (AO) or with a side focus knob. In either case, parallax is reduced or eliminated by adjusting the relative positions of the objective lens and the focus lens, as discussed above. Parallax is seldom an issue at low power, but can become problematical at higher magnifications.
The last topic I want to touch upon is not strictly speaking an optical property, but a property of ballistics, specifically bullet drop. The concept of Holdover relates to adjusting your aim to compensate for bullet drop at greater ranges. Remember that even with a variable power scope, your rifle and scope are zeroed in at a particular range, changing the magnification to see farther does not change the range at which your scope is zeroed. In order to properly determine your holdover you need to have a fairly good idea of the range of your target and a working knowledge of the bullet drop tables for your weapon and ammunition. A good Mil-Dot first focal plane reticle is an awesome tool in estimating range and holdover.
I hope that this article helps to demystify rifle scopes a bit. Remember that the terminology is just words and words can never hurt you. In fact learning the words and how they relate to a rifle scope can help you narrow down the search when it comes time to buy that perfect rifle scope for any situation.
