There are often many posts or questions about riflescopes. In order to be of service, I have compiled this information thread to answer your questions. This will be an ongoing process and some data may change or be added - e.g. the spring-piston air rifle scope section is a new addition. I hope it covers most of the subjects that you might want to know about scopes and optics. While I do have extensive experience with hunting scopes gathered over the years, I mainly drew information from expert sources such as: John Barsness, Wayne Van Zwoll, and Ron Spomer - optics editors for RIFLE magazine, Jacob Gottfredson, optics editor for GUNS magazine, Todd Spotti in the IHMSA News, and other industry sources like Burris, Leupold, Bushnell, Simmons, SWFA and others, including fellow Moderator Onepoint who assisted tremendously in input and finding information. Certainly his optic expertise should be credited on this forum.
While I tried not to bring any personal prejudice into the thread, I own and therefore am most familiar with certain brands of scopes so some of that may show. I hope you find this information most profitable in any future scope purchase, the use of your current scopes, and you will learn something of value.WHAT THE NUMBERS MEANA scope designation often is presented like this:
3-9x 40mm. Or 4x32mm
Broken down, in the first case this means a 3 power (
the “x” for “times”) to 9 power magnification variable scope with a 40mm objective (
front) lens. The size of the front lens indicates some light gathering value and helps calculate Exit Pupil (
described later). The second case would be a 4 power - fixed power scope with a 32mm objective lens. Occasionally this designation might be followed by something like “AO”. This means it has an Adjustable Objective that can be focused at a certain range to eliminate parallax. This can be spotted by a series of yardage or meter numbers on an objective that can be turned to make the numbers correspond with a line.
As of late, you might see a scope described as a “30mm” 4-12x 42mm. This would indicate that the body of the scope follows the European pattern of a larger than the American standard 1 inch body diameter and would require larger 30mm rings. Some older cheap .22 rimfire scopes, and even ancient hunting scopes, were only ¾” or 7/8” in tube diameter.
So a 4.5-14x 50mm AO would be a 4 ½ to 14 power variable scope with a 50mm objective lens and an adjustable objective. A 1 inch tube (
and ring) size would be assumed because there is no 30mm designation.
There are also a couple of other "letter" designations that may follow the numbers, and these also deal with a specific scope usage.
"XER" - E
xtended
Eye
Relief. "EER" -
Extended
Eye
Relief. "IER" -
Intermediate
Eye
Relief. There may be other brand-specific designations for a scope that has the eye relief is much longer than a regular scope and the scope is mounted further from the eye. These are used primarily in handgun applications, and some models as a "Scout" scope, which are mounted on a rifle forward of the receiver over the barrel area as popularized by Col Jeff Cooper for his "Scout" rifle concept.
TERMS & MISCELLANEOUSEYE RELIEF (ER)The distance a scope can be held away from the eye and still present the full field of view.
Some things to know about eye relief is that many, but not all, variable scopes sacrifice eye relief (
or eye relief shrinks) as power increases. The greater the top-end power, the less eye relief at those settings. Those that do threaten to give you the infamous half-moon scope cut on your eyebrow during recoil. Aim for a minimum of 3 inches of ER at the .30-06 level of recoil, 3.5 inches with .300 magnums, and 4 inches would be better. Fixed power scopes have a fixed eye relief distance. Mounting position on the rifle in the rings can be affected by ER limitations.
You can measure eye relief in your scope by gently placing a small flashlight against the objective lens and shining it into the scope. Next you hold some flat object (your palm will do) about three inches behind the eyepiece. A dot of light will be "projected" on your palm. This is the exit pupil of your scope. Move your hand until the dot is as small and sharp as possible, and the distance from the rear of the scope to your hand is the eye relief. You can measure this distance with a ruler, or you can make a cardboard tool that includes both a flat surface for the image of the exit pupil and a short ruler. Measured eye relief may not be the same as the scope maker advertises. There can be some manufacturing tolerances, but when the scope only has 2.75" and the spec sheet says 3.5" it would be enough to make a real disgruntled .300 Magnum shooter.
FIELD of VIEW (FOV)The side-to-side measurement of the circular viewing field or subject area. It is defined by the width in feet or meters of the area visible at 100 yards or meters. A wide field of view makes it easier to spot game and track moving targets. Generally, the higher the magnification, the narrower the field of view.
THE OPTICAL TRIANGLEWith scopes, there is an “optical triangle” that comes into play consisting of Magnification, Field of View, and Eye Relief. When you increase or reduce one side of the triangle, it affects the other sides. That is, increased magnification reduces eye relief and FOV; increased FOV at a given magnification reduces eye relief. Increased eye relief reduces FOV at a given magnification etc. Magnification is the biggest factor in FOV. Many scope brands tend to be slanted to one side of the FOV/eye relief triangle. This is good because it allows the consumer to pick which is most important for the particular application. A bigger FOV may help you get on target quicker, while longer eye relief may be necessary with a harder recoiling rifle. A larger diameter ocular lens also contributes to a larger FOV.
OCULAR LENSThe lens closest to your eye or Eyepiece. This part controls both FOV, as mentioned above, and focus. On many scopes the ocular housing can be turned to focus your scope for your eyes. Most are focused so they look sharp at the close distances you look through in the store when comparing and buying! This usually needs to be changed for the real world. There is a narrow lock ring at the junction of the ocular housing (
or eyepiece) and the scope body, just behind the power ring of a variable – if there is no lock ring, the scope is an eyepiece focus ring type.
1: Loosen the lock ring (
if it has one) and rotate the eyepiece counterclockwise five times. (
Warning: Do NOT turn the eyepiece more than 15 rotations counterclockwise or this can break the waterproof seal)
2: Look through the scope toward the sky, or at a white wall about 10’ away. Rotate the eyepiece, or eyepiece focus ring clockwise until the reticle appears sharp and black at a quick glance. Do NOT look through the scope as you turn the eyepiece, as your eye will adjust to the out-of-focus condition. Glancing through the scope will immediately reveal the reticle as distinctive and black when it is properly focused.
3: Lock the eyepiece by tightening the lock ring. Some models (
Like the Burris FFII) have an eyepiece focus ring and no lock ring is provided or necessary. Forgetting to lock the ring can result in inaccuracy when shooting as well.
RETICLES“Reticle” is a fancy name for the crosshair or any other aiming point (
post, dot, etc.) For years the most popular scope reticles were the crosshair and the post-and-crosshair. European scopes, including Russian sniper types, often favored a post setup of some type. In 1962 Leupold introduced the Duplex, a crosshair with heavy outer sections and slim lines in the center. This makes sense. The thick shank grabs your eye, directing it to the middle of the scope, where the thin intersection allows you to aim precisely. In addition, knowing how the wire subtends at a set magnification and yardage, you can use the Duplex as a rangefinder. This “plex” style is now the overwhelming choice of most scopes today. Variations include a heavy plex that is superior in low light, and a plex with small unobtrusive marks below the intersection that are used as aiming points or even rangefinding points, at longer range.
The more complicated military type is the “Mil-Dot” oval dot - while the civilian hunting version, a series of small lines, was popularized by Burris recently in the FF II line as the “Ballistic Plex”. Other scope makers offer a similar plex but use another term such as Leupold's "Boone & Crockett" or Nikon's "BDC" reticles. This is a very nice bonus and becoming deservedly popular. A ballistic chart, in the form of a sticker, matching a given caliber is provided, or you can do your own experimentation and make your own chart. The Ballistic Plex type requires your rifle to be sighted in at either 100 or 200 yards depending on your cartridge and bullet selection. You set your scope on (usually) the HIGHEST magnification and either used the supplied stickers/data, or make your own by actual firing at each 100 yard increment for your own very precise table.
Examples of new-style multi-reticle systems:Mil-Dot reticle:
Burris Ballistic Plex reticle
(Burris also offers other variations - more on this later)
Nikon BDC 600 reticle
TIP: Shooters must remember that in our common Second/Rear focal plane scopes (
more about this below) that these ranging marks like the Boone & Crockett are only accurate when the scope is set at the correct power (
should be in your manual, but you can email or call their techs). In other words, if the scope is turned down to its lowest power, you shoot over the top of whatever you're aiming at short of a barn.
The Mil-Dot system is very good, but requires quite a bit of learning to really make proper use of its milliradian. Based on a 360 degree circle system, a milliradian is about 1/17th of an inch. You divide the target’s height in mils by the number of interstices subtending the target to get the range. Well, you get the picture…
The plex isn’t the beginning and end of reticles. There are many varying types, such as the post, the dot, the cross-hair-and-dot, etc. Most of these are for more specialized uses, and some are generic to the scope/type/country of origin. Russian sniper scopes, for example, have there own type of reticle and there is no option for others. There are also illuminated reticles for use in poor light conditions. Most rely on a battery of some type and brightness can often be adjusted.
Mechanical reticles are made out of something like flattened platinum wire or ribbon wire suspended on a mount. Another way to make a reticle is to use a photo-etching process on metal foil. Proper tension of this type is very critical because a slight error either way can result in failure under recoil or failure with extreme temperature variation. Pointing this type at the sun can also burn them out. This type is done offshore because of the nasty chemicals used during etching. The very thinnest crosshairs are fashioned from spider web!
The reticle can be placed in two different positions in the scope:
First or
front focal plane reticle placement is common on European scopes. These scopes have the reticle placed in the objective end, or forward of the erector system. This causes the reticle to be magnified at the same rate as the target. When you increase the magnification the reticle will continue to cover the same amount of the target as it did at low magnification.
One advantage of having the reticle in the first focal plane is with a multi-point mil-type reticle because it stays the same size relative to the target.
Most
American made scopes (
including those made for U.S. companies in the Far East), have the reticle placed in the
second, or
rear focal plane. In this position, the reticle will not be magnified at the same rate as the target. In other words, when you increase the magnification on a rear focal plane scope, the reticle will subtend less of the target than it does at low magnification. For the majority of hunting applications in the US, positioning the reticle in the 2nd focal plane is desirable. The biggest drawback to our common second focal plane scopes is that on variables, they are susceptible to POI shift as the power is zoomed. Careful construction helps eliminate this, but it is not an issue with first focal plane scopes as that part doesn't move when zooming.
Most
European scopes have the reticle in the first focal plane and the reticle will remain constant, however quite large on high magnification. For most American hunting applications it is too large. Most European hunting is not done at such long distances.
Many tactical groups prefer front focal plane designs because common tactical reticles serve a dual purpose: a point of aim and a means of measurement. Reticles such as a mil dot are based on a specific subtension and require exact feature spacing to be accurate; if this type of reticle is used in a rear focal plane design, the scope must be used on a single, specific magnification (
typically high power). Placing this type of reticle in a front focal plane design allows the operator to use the scope on any magnification while retaining the exact spacing of the reticle features.
If the reticle cell has shifted, due to time and recoil, it will create excessive parallax, meaning the reticle apparently wanders around the aiming point when you shift your head behind the scope. Groups tend to string up and down when this happens. The reticle cell can also come loose. I have had this happen to the reticle on a cheap Chinese SKS scope which turned the reticle into an “X” after a minor amount of use. You can’t hit anything when this happens either!
PARALLAXA condition that occurs when the image of the target is not focused precisely on the reticle plane. Parallax is visible as an apparent movement between the reticle and the target when the shooter moves his eye anyplace but dead-center behind the scope's field-of-view - or, in extreme cases, as an out-of-focus image.
Most center-fire riflescopes under 11x are factory-set parallax-free at 100 yards; rim-fire and shotgun scopes at 50 yards. Any scopes over 10x requires some sort of method to eliminate parallax. These scopes usually have a special range focus (adjustable objective or AO) to adjust for parallax. Newer and tactical versions often feature a side-adjustment, normally placed on the left side of the scope opposite the horizontal turret adjustment. Parallax is really apparent when you try to focus on a target with a standard rifle scope at 25 yards. The crosshairs seem to “float” around as you slightly move your head/eye behind the scope.
As mentioned in the section above - if the reticle cell has shifted, due to time and recoil, it will create excessive parallax, meaning the reticle apparently wanders around the aiming point when you shift your head behind the scope. Groups tend to string up and down when this happens.
POINT OF IMPACT (POI) / POINT OF AIM (POA)The spot where the bullet actually hits the target is the Point Of Impact. The place on the target where the crosshairs are placed is the Point Of Aim. You adjust both Windage (
the horizontal left-to- right movement) and Elevation (
the vertical up-and-down movement) to Zero the riflescope at a certain distance. These is usually done by moving the adjustments a certain amount of “clicks”, which then moves the POA to coincide with the POI.
SCOPE CONSTRUCTIONConstruction will make keep that new scope reliable and working right way down the road. It will make sighting-in easier and lessen the chances of the “big one” getting away because of scope failure. It will hold up when recoil levels equal or exceed that of the .30-06 for years. It will keep you from fogging up. It will keep your zero – zeroed after the normal knocks of carry and use. It will save you from buying new scopes down the line and going through the re-mounting and re-zeroing all over again.
General scope parts (Bushnell)
Detailed scope parts and image path (SWFA)
TUBE Tubes with thicker walls and less joints resist stress forces that come with mounting and knocks. One piece main tubes are best here. As mentioned, there are both the U.S. standard one inch (25.4mm) tubes and the European-derived 30mm tubes. The main advantage to the 30mm tube is that is has more available windage and elevation adjustment range for extreme cases.
NITROGEN GAS The scope is filled with dry nitrogen gas to eliminate moisture and fogging. Quality of the seals used here is very important, because nowadays, most scopes are nitrogen-filled, but can leak due to poor seals. Variable scopes with their moving power ring are harder to seal than fixed power scopes. Quality sealing with longevity in mind adds some cost.
This is why cheap fixed power scopes may have much less of a tendency to fog compared to their inexpensive variable power brethren. A scope contracted to be built to spec for a scope “maker” in one of the two Japanese optic factories has 8 levels of waterproofing to choose from. Target price of the intended product drives the choice of “how good”. Something to keep in mind anyway.
The newest technology in this area is to use Argon gas, or an Argon/Krypton gas blend rather than dry nitrogen gas. Claims are that Argon gas does not absorb water or react chemically with water. It maintains its protective properties over a wider range of temperatures. And lastly, Argon gas does not diffuse as quickly as other elements, extending the service life of the optics longer. Of course you might have to pay a bit more for this new technology at the present time. For example, Leupold offers Argon/Krypton gas in it's more expensive VX-3 line of scopes.
ERECTOR TUBEThe interior lens assembly that is actually moved when adjusting the windage and elevation is held in place by springs. The strength of these springs determine how well a scope may stay in adjustment or “zero.” Some scopes may even feature double springs, and Burris has gone to the expense on some of their scope to incorporate a “Posi-Lock” feature that solidly locks in your adjustments with a retractable steel post. This keeps a scope from failing when subjected to heavy recoil.
These springs can go bad. If the spring that controls vertical adjustment has gone bad, for example, the groups will string very vertically, sometimes a foot or more, with no sequential pattern.
SIGHT ADJUSTMENTSQuality scopes may use steel-on-steel adjustments that are more positive and repeatable click adjustment. Cheaper ones use plastic here. While quality “name” scopes usually use good stuff, there can be issues with plastic, depending on grade and cost, either right off the bat, or down the road a few years with adjustment repeatability. Higher end scopes like the Nikon Monarch series use plastic, but repeatability seems to be fine for normal hunting use. The budget “Mart” loss-leader scopes would be suspect. Burris and Leupold VXIIs (
and up) use steel-on-steel, to name a couple. Some scopes still use a friction non-click adjustment. If done well, this can work fine for normal hunting sight-in. Leupold’s older Vari-X II and new VXI and Rifleman line use this method successfully although a click system definitely spoils you.
Another factor to consider here is the amount of clicks per inch in the adjustment. Some just give you 1 click to move the POA ½” at 100 yards. Others may give you 1/3” per click. Very common is the ¼” per click – which translates to 4 clicks will move the POA 1” at 100 yards. Some have 1/8” clicks, which is almost too fine for most uses. Some scopes will give you a “witness mark” ring of numbers on the outside of the adjustment that can be turned to put the “0” at your final zero adjustment. Then if the need arises, and you have to adjust again, you can easily return to your original setting (
marked 0). A nice, but not necessary feature.
One thing to be aware of, is that sometimes the adjustment value of each click doesn't match the claimed adjustment value. This can even extend to the vertical click being a different real-world value than the windage click. For example, a recent test of the nice Bushnell 6500 Elite 4.5-30x50 showed the value of the elevation clicks to be right on specified 0.25", but the windage value was actually 0.035". Another brand scope's actual click value was 0.28" rather than the specified 1/4". Even if they are off the specified value, it really isn't a big deal as long as they are consistent and you know the actual value - especially if you use the adjustment to crank on extra elevation when shooting at different yardages. You should probably check this out on paper first, rather than merely taking the manufacturer's word for it.
Reticles come from the factory set in the center of the adjustment range. Usually the further from the center you adjust them from center, the more erratic or "off" the adjustment value becomes. A 1/4 MOA click may not always adjust as advertised. Fire a shot - the impact may be an inch high. Turn the turret dial 4 clicks and fire another shot. Impacted 1 inch low, and so on. This can be the frustration you get when the reticle is close to the edge of the adjustment range. So you should make a serious effort to mount the scope, shimming if needed, or using something like the Burris Posi-Align rings.
TIP: Gun scribe John Barsness says that the erector tube lags somewhat behind a retreating adjustment screw. After a major adjustment, sometimes the erector tube even slides a little to one side. This is why many rifles often don’t shoot exactly to where they should after a major adjustment: The erector tube hasn’t settled into its final resting place. Over-adjusting is the easiest way to settle the erector tube, far cheaper than firing a “settling shot” and much more reliable than tapping on the turrets. Just go a few clicks past the theoretically correct adjustment, then turn back those few clicks. It doesn’t matter if you over-adjust two or three or four clicks, just make the number consistent.
Gun writer Norman E. Johnson says to always complete the windage and elevation adjustments with a CLOCKWISE screw thrust. This pushes the screw firmly against the erector tube, as opposed to the return springs doing this function. You will always arrive at a more precision adjustment in this way as you obviate the effects of a sluggish leaf spring thrust. If you want to go up or right with the reticle, simply go past your desired setting and turn the dial clockwise for final adjustment. This maneuver works with nearly all scopes regardless of price or quality.
This is the surest way to have the adjustments end up right where you want them!
MAGNIFICATIONThe most personal factor in the choice of a scope is the magnification, or magnification range in the case of a variable power scope. Way back when, snipers used 3.5x scopes, hunters used 2.5x in the woods and 4x for open country. Now the variable power 3-9x is king and shooters are increasingly getting scopes of ever higher power, usually in some type of variable outfit, which seems to reflect the American idea of “more must be better.”
While magnification may bring the object closer, the additional power magnifies and flattens the environment as well as the atmosphere between the viewer and the object being viewed. Air density and mirage near the surface of the earth are greatly enhanced, destroying the fine resolution and contrast of which the scope is capable. The detail seen (
and for binoculars and spotting scopes, the fine focus available) at a lower power, quite often render a much better understanding of the object viewed.
The advantage to the lower power scope is a wider field-of-view. This can be critical in the woods, or with running game. They also tend to be very bright. When the target is stationary and a longer ways away, the higher Xs are nice to have. Drawbacks include a much smaller FOV, and less brightness at higher settings. Secondly, running game is harder to hit and the extra magnification amplifies your unsteadiness – sometimes to a point where it is unnerving! (
You always have that unsteadiness, you just can’t see it!) The 3-9x covers most of the range that most shooters and hunters use. Because of the huge number sold, they are usually cheaper than any other variable power range. However, if your shots are up close, you might consider trading a few Xs off the top for a much larger FOV in a 2-7x. An example of this comes from the Burris specification sheet and is representative of other brands as well. Their standard 3-9x has a FOV of 33 feet at 100 yards at 3x. Going down just one power notch, the 2-7x gives you a
third bigger FOV, or 45 feet at 100 yards at 2x. This could be very critical in close woods or running game shooting and worth the extra cost over a standard 3-9x. Lastly, on a hunting rig, the lower power variables are lighter and smaller on a rifle you may carry all day. So give this stuff some thought before buying.
Scopes in the 4-12x range, and above, are really slanted either to dual purpose varmint/big game use, or varmint/target use. The newer 4.5-14x range is popular for varminting because, over 14 power, mirage often makes it look like you are look through a swimming pool at the target. Consider that even 7 power makes a deer at 700 yards look as if it was only 100 yards away. Consider that you begin to give up wide FOV on the bottom end. How many can’t see a deer at 100 yards well enough to shoot?
Snipers have also gone the higher magnification route, with a simple, reliable, fixed 10x being popular. Variables are making large inroads here, due to their increasing reliability and the ability to either go higher or lower for specific shooting situations.
As previously mentioned, magnification is very personal, just think through the intended usage and look at some of the other factors as well before making a final decision.
Zoom or variable magnification scopes have become so universal that decent fixed power scopes are becoming rare, despite their advantages of better anti-fogging, simplicity, lighter weight and smaller size. They have become a specialty or nostalgic item. One place they really stand out nowadays is on a .22 rimfire. A four power scope still offers everything needed on one of those rifles.
Drawbacks to variable zoom magnification scope would include, a larger scope, more weight, more chance for waterproofing to fail, the possibility that you may inadvertently have it on the wrong (
usually too high) power setting when a close or running opportunity presents itself. This human-based problem can be cured with discipline, but it is an entry point for Murphy’s Law!
Many scopes have a shorter amount of eye relief on the higher power settings, as mentioned in the Eye Relief section. You could place one on your .300 magnum and end up "scoping" yourself at 10x. This is mostly an issue of scope placement and picking a scope with enough eye relief on high settings to begin with.
Due to the placement of the reticle in the second focal plane, most zooms are also often prone to change POI when changing the power setting. More expensive scopes go to great lengths to minimize or eliminate this tendency in construction and European and some other scopes place the reticle in the first focal plane to eliminate this (
albeit with the thickening of the reticle when power is increased.) Less expensive scopes make it more of a crap shoot sometimes, so a fixed power is always the very safest bet with them.
TIP: When changing power on a variable, a 3-9x for an example, start at the bottom power, 3x and always go up to the desired power, say 5x. When changing back to a lower power, go all the way to the top (9x), then down to the bottom (3x), and then back up to the next desired power (
e.g. 6x). If you want to stay on the bottom (3x) it is best to go to that power and add a fraction of a turn upward. This is all it takes for most zoom systems to track properly. This minimizes POI zoom shift with decent scopes. It works.
HUNTERS WHO USE HIGH MAGNIFICATION SCOPES TO IDENTIFY TARGETS, OBJECTS AND PEOPLE ARE DANGEROUS. GET A BINOCULAR.WEIGHTIs it a factor? The more power, and thus more lenses and bigger objectives may make a scope that unbalances your hunting rifle, making it rather top heavy. This is less of an issue with a heavier target or varmint rifle. On very heavy recoiling big bore rifles, a heavy scope has a lot of inertia due to the weight, and can have more problems with recoil failure than a quality lighter scope. If you are building a lightweight “Mountain” hunting rifle, a lighter scope can shave off more ounces less expensively than any other method.
SCOPE BRIGHTNESS AND LIGHT GATHERINGLENS SIZEIt’s not all about the size of the objective lens. Riflescopes with monster objectives may actually transmit less light than much smaller instruments.
Yes, the big front window does let light in, but it’s the size of the exit window that determines how much of that light gets into our eyes.
The quality and quantity of anti-reflection coatings enhance or degrade brightness too.
How does it work?
The objective lens lets light/image in.
Several internal lenses pass that image on to the eyepiece where it is additionally magnified and passed out to our eyes.
The higher the magnification, the smaller the exit window, commonly called Exit Pupil (EP.)
The scope’s EP corresponds to our pupils, which dilate to a maximum of 7mm in a young person, declining to about 5mm by age 50 and another millimeter each decade after that. If a scope’s EP exceeds the diameter of ours, the rim of extra light spills out onto our irises, wasted.
You can see the EP of any optic by holding it at arm’s length toward a bright area and looking at the eyepiece lens. That little bright circle is the EP. Through it pass all the waves of light making up the image of that deer you’re hoping to shoot. If you have a variable power scope, turn the power ring and watch the EP change diameter. Reduce power and the EP enlarges. Crank power up and it decreases.
EP is calculated by dividing objective lens diameter by magnification.
Thus, a 40mm scope at 4x gives a huge 10mm EP. Cranked up to 10x, the instrument still delivers a 4mm exit pupil. A 4mm EP provides more than sufficient brightness for targeting a deer at one-half hour after sunset on a cloudy day in the woods (
or legal shooting hours most places). An optic with an exit pupil of 1.3mm restricts light transmission to such a degree the optic's effectiveness is rendered almost useless.
As an illustration of how all factors optical are co-dependent, magnification and image quality come together at a specific exit pupil diameter regardless of the much advertised twilight factor. When a combination of objective lens and magnification dips below 2.5mm, image quality suffers substantially. Above 2.5mm, image quality becomes more apparent, as each increase in exit pupil diameter enhances resolution and contrast.
LENS COATINGSAnti-reflection coatings contribute to brightness by managing the light efficiently.
Raw optical glass reflects about 4 to 5 percent of the light that strikes it and as much that passes out of it. The loss occurs at each air-to-glass surface, so a scope with seven lenses could lose 70% of the light. Adding insult to injury, the reflected light bounces willy-nilly, lens to lens within the scope, appearing as a foggy haze that reduces contrast and obscures the principal image.
Way back when, optical engineers at Zeiss discovered that coating lens surfaces with exotic coatings could reduce reflected and air-glass losses. Eventually they found that multiple coats of different exotic compounds really helped out. Thus the coated and multicoated lens was born.
Definitions to learn when shopping for scopes:
Coated lens – A single layer on at least one lens.
Fully-Coated – A single layer on all air-to-glass surfaces.
Multi-Coated – Multiple layers on at least one lens and all surfaces are coated at least once.
Fully Multi-Coated – Multiple layers on all air-to-glass surfaces. This gives maximum performance/brightness. This is the best.
These anti-reflection coatings reduce reflection loss to as low as .2% per surface (
claimed by manufacturers anyway.) Let’s be generous and give them .5% at each surface. That totals 7% reflection loss in the above hypothetical 7-lens scope for total light transmission of 93%. Whether the manufacturer’s numbers are correct or not, we do know that multiple layers of these coatings, preferably on every lens surface, are our best defense against reflection loss and internal flare and glare. This is good news because, although multi-coatings do add cost, they don’t add bulk or weight or require unusually high rings. It’s almost an optical free lunch.
One exterior lens coating that offers both optical improvement and and all-important lens protection for those highly refined lens surfaces and exotic coatings is Leupold's DiamondCoat 2. This is an ion-assist lens coating, for higher light transmission and the greatest level of abrasion resistance Leupold has ever offered. DiamondCoat exceeds military standards for hardness and durability. Doubtlessly, in the competitive scope and tactical scope market, other makers are coming up with similar coatings for their more expensive models.
There are also lens coatings that do not contribute to the optical quality of the scope, but rather aid other concerns. The first of these is a hydrophobic coating. This is a exterior lens coating that repels water. Bushnell introduced this coating technology and calls it "Rainguard". Raingard doesn't prevent all moisture from sticking to the lens, but does repel enough to provide an image usable for most shooting.
RESOLUTIONAnother contributor to a bright scope is resolution. Resolution is one of two premier requirements of most optics. We see this as a crisp, sharp image that jumps from the background. It is created primarily by lens quality, a precise grind of clean glass that focuses a perfectly sharp image without significant aberrations in lines and colors. Remember that the better glass costs money. Above 10x, chromatic aberrations begin to soften edges, because not all colors in the spectrum can easily be focused back to the same point. The result is sort of a double image , one laid over another that is slightly larger. The fringe color is usually yellow or green. Apochromatic, HD and ED lenses largely correct this color fringing. It also takes adequate transmission of light to achieve good resolution.
CONTRAST Contrast plays a role here too. It is the second premier requirement of most optics along with resolution. Contrast is the apparent difference in brightness and tone between a tan antler time and a brown tree limb. Anti-reflection coatings enhance contrast by minimizing glare that mutes contrast. Lens quality, its ability to efficiently pass certain wavelengths of light (
especially in the blue spectrum where human eyes are least sensitive, i.e., during twilight) plays a big role in boosting contrast too. A scope that transmits an extremely sharp, high-contrast image can look noticeably brighter than another that actually passes more light, but poorly resolved.
PLASTIC LENSESAs mention earlier, good glass is expensive. There are only a handful of companies that make the glass to buyer specifications for virtually all the optics companies. Yes, even our U.S. firms use Japanese glass. As a lower cost alternative, some outfits are using plastic lenses. It has the virtue of being lighter and cheaper, however, it is also softer, so it cannot be used for an outside lens surface that might get scratched. It also tends to scatter light rays and has poor color correction, scattering colors. Plastic can be molded into aspherical shapes that can produce a wider FOV and flatter images at the fringe. This is what Simmons does with its AETEC series. The lens is internal in that application. You can probably safely assume that the least expensive scopes make the greatest use of plastic lenses.
SCOPE SUMMARYHow much any of this matters is for each shooter to determine. At some point, improvements in optical quality become difficult to detect. Not all eyes are sensitive enough to see subtle differences, and if you can’t see it, why pay for it? And how much of this is really necessary anyway? Money being no object, we’d all buy the best. But when money is an object, we must compromise. Noted gun writer and columnist on optics Wayne Van Zwoll says that he begins that compromise by "recognizing that a scope is a glorified front sight. As long as it keeps its reticle where the barrel prints bullets, shot after shot, season after season, I can live with slightly less optical quality than required to observe the moons of Neptune." I would second that opinion.
If you are a hunter, you might want to reserve the big optical cash for high quality top-line binoculars and/or spotting scopes first. That’s what finds the game. A decent scope with as much of the above qualities, plus reliability, rain or shine, year in and year out, even with 300 Magnum recoil, can easily be found in the $200 to $400 range. Buying one for less than that requires a careful look into construction, coatings, quality of glass, repeatability of adjustments, etc. There will be more compromising on the lower end of the cost scale. Think carefully about intended usage and magnification.
What about Tactical scopes? Patrick Sweeney of the Front Sight Firearms Training Institute and gunwriter as well, has what he calls his "Good-Enough" law: "Yes, the military buys $1,200 scopes for rifles. Heck, they buy
$12,000 scopes. Why? Because it costs more to helicopter the SpecOps team to their ambush site than you'll spend all year on guns, ammo, gear and flavored coffee beverages. The cost of a scope is nothing to the military; it's less than the HV/AC budget for the Pentagon building. If you have enough money that the cost of a scope is nothing to you, have at it. Otherwise, a $400 scope and $800 in practice ammo will serve you better in the long run."
Many of the practical among us want a do-it-all rifle and scope, a "universal" scope if you will. Inconvenience is the downside to using "universal" tools. It isn't always pleasant putting up with oversize equipment for small jobs in order to have adequate equipment for big jobs. But 3-9x and 4-12x zooms come fairly close. That is why 3-9x is the biggest seller (and thus cheaper).
And lastly, remember that those big 50mm “Light Gathering” objective lenses are basically hype, require extra high rings, degrade proper cheek weld, and make the scoped rifle top-heavy and ungainly. So beware!
SPRING-PISTON AIR RIFLE SCOPESPowerful spring piston airguns can tear apart an ordinary rifle scope.
To understand why this happens, we need to examine some basics about recoil and spring-piston airguns. First of all, it’s hard to imagine that a spring-piston type airgun would have any recoil at all. As we all know, with normal firearms recoil is generated by the bullet and gases exiting the muzzle. As you increase the weight and speed of the bullet and the speed and volume of the gases, recoil will increase. The only thing mitigating the recoil generated is the weight of the firearm. Heavier guns recoil less than light guns all other things being equal. All recoil is generated to the rear.
On an airgun, a typical pellet will weigh only a minuscule 7.9 grains, and velocities are usually well below 900 fps on most rifles, and less than 500 fps on pistols. Spring-piston rifles and pistols are complicated, heavy mechanisms and consequently often weigh as much as a normal firearm, or more. Therefore you could reasonable expect the recoil generated in them should be absolutely negligible. With that in mind, how was it possible then for a first-quality rifle scope that will hold up to the recoil of a .300 Magnum to be ripped apart by a spring-piston air rifle that doesn't kick at all?
Unlike regular firearms, spring-piston recoil is NOT generated by the pellet and the air being expelled out of the muzzle. It’s actually being generated by the
piston and spring, both of which are very heavy components, especially the large steel spring. When the sear is released, the highly compressed heavy spring will jump forward with tremendous force pushing the piston ahead of it. As the spring and piston are moving forward, the gun is recoiling back against your shoulder with significant pressure. Now here’s the part that many people don’t understand. As the piston comes to the end of the compression chamber it will actually strike the wall with substantial force, and the gun will now bounce forward, recoiling away from you. In addition, this action generates vibration in all directions. Even if your spring-air rifle is "recoiless" like an RWS 54, that only means that they do not cushion themselves on the shooter's shoulder but have recoil forces within the rifle transmitted to the scope, if not the shooter.
So the essential elements to be remembered here are that recoil from a spring-piston airgun is not light but actually fairly considerable, and that there are two recoil pulses in opposite directions i.e. one to the rear and one forward. For a scope to withstand the recoil pulse away from the shooter, it has to be constructed specifically for that task. One thing a spring air rifle scope must have is a reticle braced both front and back.
Many scopes fail after a limited number of shots, sometimes as few as 20. Mostly this shows up as a reticle sitting loose or pointing in the wrong direction in the sight picture. Sometimes the reticle adjustment mechanism fails. Sometimes the warnings come in a more subtle manner in that point of impact changes all the time for no apparent reason.
Therefore, with a scope designed for a normal firearm may, or may not, have the reticle braced front and back. It varies from brand to brand, even perhaps from model to model. It is less likely that the bargain-priced rifle scopes feature this. Therefore some standard scopes will hold up to a spring-air rifle without issue, while others will fail. Even some expensive firearm scopes may fail miserably on a spring-piston air rifle. One cannot use any blanket statement that includes "all" or "none" on this issue.
Because the problem is well known in airgunning circles, actual quality air gun scopes are made to deal with the unique demands of the spring piston air rifle. So it lessens the crapshoot factor. You should check the scope specifications and warranty provisions before buying.
Besides being constructed specifically to withstand a forward and rearward recoil pulse, what else distinguishes an airgun scope from a normal rifle scope? Actually the most important characteristic of an airgun scope these days is its ability to focus down to 10 meters parallax free. Ten meters is the standard distance that most airgun competitions are held. Most varmint shooting is done under 50 meters. So you absolutely need a close-focus scope that is parallax-free at these unusually close ranges.
In today’s world, the distinction between air gun scopes and rifle scopes has become not only blurred, but very jagged as well, depending on the manufacturer. As it turns out, many of the main stream scope manufacturers have substantially re-engineered their construction designs allowing at least some or even all of their rifle scopes to be used on a “springer” air gun. Leupold, Burris, and Bushnell (
except for their bottom 2 economy models) say that
all their scopes are safe for spring-piston airguns. Weaver, Simmons, BSA, and Tasco say that not all their models are safe for spring-air rifles. All of the manufacturers listed offer designated airgun scopes. However the kicker here is that just because a rifle scope can also be used on a spring-air rifle, that doesn't mean they are ideal. The problem is a lack of parallax adjustment down to 10 meters, which is so critical for accuracy in these basically short range rifles.
An air gun scope is therefore defined as one with special construction features to handle the double recoil of a spring-piston airgun - and which has also been designed for parallax free viewing at 10 meters.
So “
what is the difference between a rifle scope and an air gun scope?” The answer is “
It really all depends on who the manufacturer is.” In the case of Bushnell there is essentially no difference between the two types. In the case of Burris and Leupold, there’s no difference in construction, but there is in optics. In the case of other manufacturers like Weaver, and Simmons, there is a definite difference in both construction and optics. For BSA, it’s a mixed bag. Some of their rifle scopes are mechanically compatible with air gun shooting and some aren't. However, none of their rifle scopes are optically compatible with air gun shooting. With Tasco, two of their rifle scopes are both mechanically and optically compatible with air gun shooting. They are the 8 X 40 X 56 Tactical scope and the Mag 40 6 X24. Interestingly, this particular Mag 40 scope is sold in the U.S. as a rifle scope and is sold in Europe as an airgun scope. The other scopes in the Mag 40 line do not meet either of the requirements for air gun shooting. Other Taco rifle scopes do meet the mechanical requirements but not the optical while even still others will correct parallax at 10 meters but not meet the mechanical requirements. You figure it out. There are plenty of scopes made especially for spring-air guns, like RWS etc., but your best bet is to check with sources that sell or cater to air rifles on the internet or elsewhere.
Decide on the quality of air rifle scope you want, then buy one a little bit nicer than that. It’s better to get something a little nicer than to get something you will regret having. The old saying, "You get what you pay for" is especially relevant with spring-air rifle scopes and optics in general.
You should also use unusually sturdy rings and mounts because the recoil and vibrations can affect this area as well - as weird as this may seem. Mounts will loosen and the scope can work loose in the ring and move. One piece mounts are usually recommend for high powered spring air rifles that have a lot of recoil. A one piece mount is more sturdy and can withstand the intense recoil of magnum air rifles.
