Scope Attributes

Scope Attributes

Let’s talk about magnification. The average human eye can distinguish approximately one inch at 100 yards or one minute of angle (MOA). A 1X scope will not improve on this but it does bring the reticle and target onto the same focal plane making sight alignment simpler. A 1X scope may also increase the light received by the eye making the image a little clearer. The scope will not cause the rifle to be any steadier but more magnification will allow us to see smaller movements of the rifle and possibly bear down and steady the rifle a bit more or break the shot when the alignment is closer.

As a general guideline I figure about one magnification per 100 yards of the maximum range expected. Therefore, a 6X scope is suitable out to 600 yards. That doesn’t mean the scope is useless beyond 600 yards. The 1,000 yard Wimbledon cup was won with iron sights a few years ago. Remember, the scope doesn’t hold the rifle any steadier, only you can do that. The scope may help you detect your unsteadiness.

As magnification increases, your ability to detect smaller details increases but your field of view decreases. A smaller field of view can make it difficult to bring the scope onto small targets. It can also make it difficult to follow moving targets before they leave the field of view. With a high magnification scope and a very close target, you won’t be able to see the entire target. If all you see is a bit of camouflage uniform, you won’t know exactly where on the uniform your bullet will strike.

Mirage can also be a problem with higher magnification scopes. Target shooters use very high magnification scopes as they allow the shooter to read mirage, follow the bullet trace to the target, and possibly even spot the bullet strike on the target. The target shooters target is stationary; this means that any movement on the part of the target that the shooter sees is due to mirage. A hunter or sniper won’t know whether the movement is due to the target or mirage so needs a lower magnification. The sniper won’t be able to read mirage through his scope but the target image will be clearer. Varmint hunting prairie dogs in the Dakotas, everyone would be doing just fine first thing in the morning when the wind and mirage were minimal. As the ground started to heat up and the wind started to come up, the mirage would become a factor. The guys with the big 24 and 36X target scopes would be the first to lose their targets in the mirage. As the day went on, 12X would generally be the maximum magnification that would still be usable. If mirage got really bad, only the guys with 8 or even 6X scopes would still be doing well.

Higher magnification scopes also pass less light on to the eye than a scope of lower magnification. Increasing the diameter of the objective lens increases the light gathered so the higher the magnification the larger an objective you need to get the same brightness of image- all other things being equal. With a 3-9X variable scope and a 42mm objective, you’ll get your best low-light image with the scope set at 6X.

Scopes come with two types of magnification- fixed and variable. The fixed scope offers only one magnification while the variable can be adjusted through a range of magnifications. Usually the high magnification on a variable is 3 to 4 times the low magnification- 3-9X, 4-12X, 6-24X, etc.

Variable scopes have more moving parts and lenses than fixed power scopes and usually more openings in the scope tube. Increased complexity means increased cost and decreased durability. Military snipers, being harder on equipment, may wish to go with a fixed power where a police sniper may find a variable more suitable.

So, for the police sniper who works at generally closer ranges and isn’t as hard on his equipment, a 1.5-6X or 2.5-8X variable scope would probably be the best choice. For a military sniper an 8 or 10X fixed would be preferable.

A tactical scope must be reliable. A tactical scope is also subject to more abuse than a target shooter’s scope so must be durable. As a general rule, the more reliable and durable a scope, the more expensive it will be.

Scope bodies can be made of aluminum, steel, titanium, or carbon fiber. Steel tubes will be the heaviest but also the strongest given the same thickness of material. With thicker materials, aluminum, titanium or carbon fiber will make a stronger scope for the same weight. The exotic materials will be the most expensive.

A one inch scope tube has been the standard in the U.S. The 30mm scope tube has been the standard in Europe for years and is gaining popularity in the U.S. The 30mm tube allows for thicker body materials, larger internal lenses with the attendant increase in light transmission, and more room for windage and elevation adjustment. Larger scope body diameters are available but the 30mm is pretty much the standard for tactical scopes. Make sure your rings match your scope tube diameter.

The scope tube can be made in one piece that is machined and drilled for the adjustment turrets or it can be made of several pieces that screw into the turret mounting. Obviously the single piece is preferable.

Your windage and elevation adjustment knobs should be adjustable without the use of tools. Click adjustments allow you to make known windage or elevation changes by feel. Target style turrets are easier to grasp and the markings are larger and easier to read although the target turrets stick out from the scope tube farther. Very large knobs on the target turrets make adjustments while wearing gloves easier. The oversized target knobs don’t allow for turret caps. Turret caps seal the adjustment mechanism against dirt and moisture. Without turret caps, the seals around the adjustment knobs become more critical to keeping contaminants out of the scope mechanism.

Scopes with click adjustments are calibrated in varying gradations of adjustment. Some common gradations are 1/8, 1/4, 1/2, or full minutes of angle adjustment per click. Metric scopes are calibrated for centimeters or fractions of centimeters at 100 meters so commonly end up at about 1/3 MOA per click. Quarter minute adjustments are pretty standard on tactical scopes. Unless you’re shooting groups in the field that are considerably less than your click adjustment resolution, finer adjustments aren’t necessary.

It’s sometimes considered desirable to have the elevation achieve its full range of adjustment in a single revolution to limit confusion as to how many turns up from the bottom the knob may be or to match the markings on a ballistic cam. The Leupold MkIV M3 series and the Unertl M40 use full MOA clicks rather than a finer adjustment on the elevation knob so that the full range of adjustment can be fitted into a single revolution.

Ballistic cams are sometimes incorporated into the elevation turret. A ballistic cam matches the elevation adjustment to the ballistic curve of your cartridge. Ballistic cams are quick and easy to use but they will only precisely match one bullet at one velocity in one set of atmospheric conditions. A ballistic cam that doesn’t match your round can be zeroed either to minimize the error over the entire range- similar to zeroing a scope for a point blank so that the ballistic curve doesn’t stray too far from the line of sight for its entire length as far out as practical. Or the cam can be zeroed at the longest range you expect to be shooting so that the errors occur at the closer ranges where they will be smallest. Scopes with standard elevation adjustments can be marked with the various range zeroes giving similar results to a ballistic cam with more flexibility.

On some target scopes the W and E adjustments are incorporated in the bases. The old target Unertls that appeared on the Springfield and Winchester sniper rifles used such a system. The Unertl scopes also slid within the mounts under recoil. Because the scope tube wasn’t drilled for the turrets or subjected to much recoil, it made for a durable scope. Redfield style bases, as well as some other systems, allow for coarse windage adjustments without using up the adjustment in the scope. The ART system incorporates a ballistic cam into the adjustable base. The ART system is very fast but a little less durable than standard mounts. The ART is also complicated to zero and it’s common to run out of internal scope adjustment while trying to sight-in if the adjustments aren’t made correctly.

The more total windage and elevation you have in a scope, the better. To maximize the amount of W and E you have available you should try to make coarse adjustments with the mounting system before you use the internal adjustments on the scope. For long range shooters you should use a down-angle mounting system so that the reticle is nearly centered at the longer ranges. If your scope has 90 minutes total elevation adjustment and the reticle is mechanically centered with a 100 yard zero, you only have 45 minutes of elevation left to make your longer range zeroes. By using a scope base that adds 20 minutes of elevation, you now have 65 minutes to get from 100 yards to your longer ranges.

Another concern at longer ranges is the way in which scopes are constructed. Your W and E mechanism is contained within a round tube. Although your scope may have 90 minutes total adjustment in both W and E, if your elevation is adjusted all the way up, because the reticle is now toward the lower edge of the scope tube your windage is going to be severely limited. Since you’re going to need the maximum amount of windage adjustment at the longer ranges, by using the base to keep the reticle centered for the longer ranges, you will maximize the amount of windage available to you.

The brightness of a scope is due to a combination of the size of the objective lens to gather light and the ability of the internal lenses to transmit light. The larger the objective, the brighter the scope; within limits. Chemical coatings are used on the lenses to increase light transmission and decrease glare. Early “windowglass” scopes didn’t use any coating and were relatively dim in comparison to the cheapest scopes available today. Coatings have advanced from single chemicals to a combination of chemicals (multi-coating) so that the light transmission approaches as close to 100% as is physically possible. Another type of coating is protective and prevents the exterior lenses from being scratched. Multi-coated lenses are a must.

Besides increased brightness, a larger objective lens offers decreased mirage. Take two 4X scopes- one with a 20mm objective and the other with a 50mm objective- the smaller objective scope is looking through a smaller window of atmosphere. A relatively small amount of mirage between the smaller scope and the target will disrupt the image. Because the 50mm scope is gathering light from the target over a broader area, it takes a lot more distortion from mirage to disrupt the image. If mirage is a problem, by turning the magnification on your 3.5-10X50mm scope down to its minimum, you will minimize the image distortion.

Exit pupil refers to the amount of light that comes out of the ocular lens and into your eye. Divide the objective lens diameter by the magnification. A 10X scope with a 42mm objective will have an exit pupil of 4.2mm. The larger the exit pupil, the brighter the scope. However, there is a physical limit. In darkness the human pupil opens up to approximately 7mm. Therefore, an exit pupil in excess of 7mm isn’t really usable to the human eye. An exit pupil larger than 7mm won’t look any brighter but it will allow the eye to be slightly off center and still get a full image. So a 4X scope with a 28mm objective is going to look as bright as a 10X scope with a 70mm objective (everything else being equal). A 3.5-10X with a 50mm objective is going to give a 7mm exit pupil when set at 7X; 7X will give the best combination of magnification and exit pupil for low light viewing.

Physical size may also be a limiting factor for an objective lens. A larger lens means the scope will have to be mounted higher above the rifle action. Many aftermarket items are designed for a specific lens size and may not be available for an oversized objective.

A sunshade can be added to the objective and serves two purposes. The first purpose is to prevent sunlight from striking the objective. Sunlight entering a scope will be magnified into the interior where it can do damage. Sunlight can also cause glare (reduced by multi-coating) and wash out the target image. Perhaps most important to a sniper, the sunlight can be reflected toward the enemy and compromise the sniper’s position. A sunshade as long as the objective diameter will prevent sunlight from striking the lens unless you’re silly enough to point your scope directly into the sun.

The second purpose for sunshades is to prevent heat waves from the barrel from distorting the target image. This is more of a concern with very high magnification scopes. For a sunshade to work in this fashion it must be nearly as long as the barrel. You’ll often see target rifles with very long plastic (sometimes clear) or paper tubes attached to the objective. An emergency sunshade can be made of a toilet paper or paper towel core.

The Anti-Reflection Device (ARD) performs the same function as a sunshade for preventing reflection or glare without being nearly as long. The ARD is made up of dozens of little honeycomb shaped sunshades. Because each cell of the ARD is very small in diameter, it doesn’t need to be 40mm long. The ARD is a must-have.

When the U.S. Navy determined their requirements for a sniper scope, they decided on having the objective lens as close to the front of the scope as possible. This was so that in the course of maritime operations the lens was more easily wiped clear of water. Most objective lenses are set back a bit to protect the lens and leave a threaded surface for the mounting of accessories.

Eye relief is the distance your eye should be from the ocular lens to get a full image through the scope. Eye relief is usually a few inches. If a scope has too little eye relief, you’ll find that the rear end of the scope will hit you in the face under recoil. Get a scope with sufficient eye relief and make sure it’s mounted far enough forward on the rifle that you won’t get tagged when firing from the prone position. Some models of scope have a rubber ring around the ocular to give a little cushion should you get kissed by the scope.

Some very expensive scopes have a critical eye relief; that is to say your eye must be a precise distance behind the scope to get the full image. A more forgiving eye relief means your eye can be quite a bit forward or back from the ideal distance and still give a full image. Leupolds were always good about having a forgiving eye relief. Nikons used to have a very critical eye relief.

Parallax is the tendency of the target image to move in relation to the reticle if you move your eye from directly in the center of the scope. A scope can only be set parallax free at one range at any given time. If you focus your scope at 200 yards it will be parallax free at that range. Most hunting scopes are focused at a single range (usually 150 yards or so) and the parallax is not adjustable outside the factory. Most target and tactical scopes have some way to adjust the parallax and focus at various ranges. The higher the magnification, the more parallax will be present. Target scopes usually have an objective that can be focused on the target eliminating parallax at that range. Because it’s awkward to reach up and over to adjust the objective when in a shooting position, most tactical scopes have their parallax adjustment either on the eye-piece (European style) or as a third turret opposite the windage adjust. To adjust the scope to eliminate parallax you can either set the adjustment to the range printed on the scope, or you can simply look through the scope and bring your intended target into perfect focus.

Parallax adjustment is desirable but not indispensable. For low powered scopes or relatively short ranges the parallax is not going to be critical. If you keep your eye centered in the scope, parallax will have no effect. With scopes that have the reticle in the rearward focal plane, simply keep the reticle perfectly centered in the field of view and you won’t get any parallax.

The factory will reset the parallax on most scopes so a cheaper scope can be focused for 300 or 400 yards and remain relatively parallax free for moderate ranges.

Don’t confuse the ocular adjustment for target focus with the standard ocular adjustment on most scopes. The standard adjustment is designed to bring the reticle, rather than the target, into focus to allow for individual eyes. The standard method of reticle focus is to point the scope at a blank wall or empty patch of sky (nowhere near the sun), look quickly into the scope, and bring the reticle into focus with the eye-piece rather than letting the eye focus on the reticle. It may take several tries to get it. The idea is to reduce eye strain by not forcing the eye to try and focus.

The reticle is the weak point on most modern scopes. I saw more broken or loose reticles than all other scope problems combined. A broken reticle is usually pretty plain but a loose reticle may not show up until your groups suddenly open up. In the past, reticles were made of spider web or horsehair (thus the term “cross hair”) or something similar. The old Unertl target scopes were actually field maintainable by opening up the scope and using tiny screws to hold a strand of hair, should the reticle break. (This meant the Unertls weren’t sealed and would fog up with changes in humidity.) Currently the cheapest scopes use a reticle that’s made of a plastic film. Most scopes use a reticle made of fine wire. The very best scopes use a reticle that is etched into one of the lenses. Obviously, the etched reticle is much less likely to break than a thin piece of plastic or wire. The laser etched reticles are also easier to manufacture to precision and make more complex reticle patterns practical.

The reticle can be placed in the forward (first) focal plane with the target or in the rearward (second) focal plane. The forward focal plane is typical of European scopes while the rearward is more typically American. Shepherd scopes use a dual reticle with one in each plane.

With the reticle in the rearward plane, as you change windage or elevation the reticle stays in the center of the field of view. With a forward plane reticle, the reticle will move up or down in the field of view with elevation changes and horizontally with windage changes; this movement can be disconcerting to someone who’s used to the American style scopes and I had several customers complain that their brand new Zeiss scopes were broken because of it.

The real difference between forward and rearward plane shows up with variable scopes. A forward plane reticle will appear to change size with changes in magnification- the reticle stays the same size in relation to the target. The rearward plane reticle seems to remain the same size while the target image grows or shrinks. This makes a difference in how the scope can be used to determine range, which I’ll get to in a moment.

The final difference in variable scopes is that a rearward plane reticle will move slightly with changes in magnification and so will your point of impact (POI). The very best scopes are manufactured to such tight tolerances that the POI change will be unnoticeable. In a cheaper scope you should consider yourself lucky if the POI moves less than 6 MOA over the adjustment range. Because the reticle is in the same focal plane as the target with a forward plane scope, there is no POI change possible when changing magnification. The forward plane scope has a definite advantage in a variable scope.

There are an almost infinite number of reticle designs floating around out there. Most have some sort of ranging capability. I prefer to avoid reticles that are too complex- too busy. Murphy being what he is, the more stuff you have in your field of view, the more likely you are to get confused. All those little hash marks and numbers may look great on a target, but when you get against a mottled target you may not be able to see them all or you may even obscure the target. Some of the more popular reticles include the cross hair, the target dot, the duplex, the German post and cross hair and the mil-dot.

The cross hair is the old standby and consists of a vertical line and a horizontal line that meet in the center. Cross hair reticles can be quite thin or very thick. A too thick reticle can obscure the target but a too thin reticle can be hard to see against a busy background.

The target dot is simply a cross hair with a dot in the center. The dot is usually 1/8 MOA across. Target dots were popular for varmint scopes some years ago until people figured out how easy it was to lose the dot against a dark background. I think most of the popularity was due to folks confusing the target dot with a mil-dot.

The duplex reticle is a cross hair with thick sections to the outside and a thin section in the center. The duplex is probably the most popular reticle for hunting scopes. The thick sections of the duplex make for good visibility against different backgrounds and draw the eye to the center of the reticle. The thin section allows precise work without obscuring the target. If the angular measurement between legs of the duplex reticle is known it can be used for ranging or hold off.

The German post reticle is similar to a duplex. A thick “post” comes up from the bottom of the scope to the center. A thin horizontal wire cuts across the center of the image. The post and cross hair is a very fast reticle and is best suited to low powered scopes. As the post and cross hair gives a similar sight picture to an aperture and blade iron sight, it presents less confusion to someone who’s familiar with the latter. Some good work can be done with a post and cross hair but I prefer a duplex.

The mil-dot reticle is simply a duplex with a series of evenly spaced dots on the thin portion of the cross hair. The dots are one milliradian or one mil apart. The mil-dot is my favorite for field work as it has all the advantages of a duplex with the added ranging and hold off capabilities of the mil-dots. Should the reticle be seen against a too busy background, the dots can easily be ignored.

Hash marks or dots can be placed on the lower leg of the cross hair to match the ballistics of your cartridge. The scope is zeroed for 200 yards and the marks are used as aiming points for more distant ranges. This type of reticle must be custom made for anything other than a standard round. The ballistic reticle is best suited to moderate ranges.

Some of the reticle patterns can be used to determine range to the target. The usual method is to compare something of known size downrange to something on the reticle that allows angular measurement. With the mil-dot scope you measure your known size object against your mil-dot scale and apply a formula. Another excellent ranging scale that’s found in the SVD scopes uses a series of stepped hash marks or a curved scale in the bottom of the scope. The target is placed under the hash marks or curve and the range is read below it; very fast and easy. As long as you know the angular measurements of each part of your reticle, even the standard duplex can be used for limited ranging and holdoff.

Any of the reticles that use angular measurement in this fashion must be of a fixed size in relation to the target image. For a mil-dot reticle to work it must be used in a fixed magnification scope or a variable with the reticle in the forward focal plane. A mil-dot in a rearward plane variable will only be the correct size at a single magnification.

Variable scopes with the reticle in the rearward focal plane allow a different method of ranging in which the target is placed between two points on the scope and the magnification adjusted until the target fits between the points. The range is then read either on a scale in the scope or off the magnification adjustment on the exterior. The old M40 system mounted a Redfield 3-9x that used this system. The ART system works the same way but as you range your target, the cam in the mount automatically adjusts your elevation.

Some scopes currently have a laser rangefinder incorporated in the scope with the attendant increase in bulk and complexity.

Illuminated reticles are available and increase the visibility of the reticle in dim light or against dark backgrounds. Illumination can be achieved with a radioactive element or batteries. There is also an accessory that mounts on the objective lens to provide illumination to standard reticles. Illumination increases the complexity of the scope.

Whatever type of scope you use, lens caps are a necessity. Lens caps protect the lenses from dirt, damage and sunlight. I find the flip-up type to be the handiest and the screw-in type to be the most awkward. See-through caps are next to useless as they provide no protection from the sun and distort the image if you try to shoot with them in place.

Oops, I forgot one more trick with a rear plane variable. Because the angular measurements of the reticle change with magnification changes it’s possible to use the lower leg of a duplex reticle, where it changes from thick to thin, as a second aiming point for various ranges. Let’s take a Leupold 3.5-10X Vari-X III with a duplex reticle. At 10X the distance from the center to the top of the lower thick section is 2.5 MOA. At 3.5X the distance is 16 MOA. That means if I zero this scope at 100 yards, I can use the top of the lower thick section for a second aiming point for an elevation gain of 2.5 to 16 inches depending on how my magnification is set. A little experimentation will show which magnification matches which zero range for my particular cartridge. The only real downside is that the closer zeros use the higher magnification and the longer zeros use the lower magnification; just the opposite of what we’d normally want.