I’m posting this write-up in response to several threads that have popped up over the last few months, in an effort to make it the one place to come to for some insights, if not answers.
Here’s the deal about optics in general and riflescopes in particular; it’s all about trade-offs and the biggest factor is money, but it’s not the only factor.
I will list the various components or factors to consider when you are looking for a scope and proceed from there. Essentially, these components make up the cost of a riflescope. I am presenting these components in what I think is a logical order, but this order is not indicative of the importance in the cost of the finished riflescope.
The riflescope tube.
The riflescope tube is the first thing you see when you look at a riflescope. This is what hold the various lenses together and keeps everything neat and clean. The tube is usually made up of a single piece shaped from a cylinder of aluminum. There are other more exotic materials used to make a riflescope, but they all add to the unit cost.
The main tube diameter is the important aspect to consider here as there are some ramifications that come from that measurement. Rimfire scopes are usually ¾ inch (19mm) in diameter. The vast majority of scopes sold in the US have a diameter of one 1nch (25.4mm). The next most popular size (mainly all European scopes) have a diameter of 30mm. Beyond that you will find scopes that have a main tube size of 34mm and 35mm and I’m sure there are bigger ones. Sometime back there were also scopes with a 26mm diameter but I’m not familiar with anything of recent vintage in that size.
Before we go further, one must realize that inside the main rifle tube sits another tube that contains the reticle, the erector assembly and the zoom lenses if this is a variable magnification scope. The knobs push the inner tube up and down and side to side, essentially changing where it looks at the image created by the objective lens (see below).
As you might have guessed the larger the diameter, the larger the cost. So what are the reasons for these increasing sizes?
The vast majority of scopes with a 1inch or 30mm tube use the same size of internal lenses, so optically there is no difference between them, provided everything else is the same. The 30mm tube will not “transmit” more light than a 1inch tube. A riflescope is NOT a water hose. (Think of the word “transmit” as “allow light to go through.”) The only way to “transmit” more light is to use bigger lenses.
The extra 4.6mm of the 30mm tube will provide more room for reticle adjustment and that translates to a wider range of MOA or Mil adjustment range. Where a 1 inch tube will provide say 40MOA of total adjustment, the 30mm tube will provide for maybe 80MOA of total adjustment. So if you want to shoot at long distances and you don’t want to bother with a canted rail, a 30mm tube is something that you may want to consider.
The larger diameter tube will also be a little stronger than the 1inch tube, even with the same thickness of aluminum.
The larger diameters like 34 and 35mm allow the use of larger internal lenses while still retaining some measure of adjustment range. These larger lenses WILL transmit more light, but when you get into these tube diameters the cost skyrockets. Also, your choices of rings, anticant devices and so on, are reduced and their prices go up also.
So: 1inch is the optical equal of the 30mm tube. The 30mm tube allows more adjustment range. The 34/35mm diameters are better optically than the smaller size due to bigger internal lenses. The cost goes up commensurate with the diameter.
A final note on riflescope tubes; the manufacturers usually replace the air inside these tubes with an inert gas, like argon or nitrogen before sealing these scopes. This makes the lenses inside fog-proof and as crystal clear as possible as nothing should get inside the tube. Don’t take your riflescope apart.
The objective lens.
The second thing you notice about a riflescope is the usual bell in the front where the objective lens sits. The size of the bell (if there is one,) is dictated by the objective lens diameter and its shape shows how the objective lens focuses the image at the first focal point. The diameter of the objective lens is THE ONLY determining factor in how much light gets into the riflescope. As stated earlier the bigger lenses transmit (see definition above) more light than smaller lenses. The objective lens, so named because it shows the “object” or the target, comes in a range of diameters and focuses the objective image into the real first focal plane. More on this later.
The objective lens has to be the very best that it can be because the image that it produces cannot be made better inside the riflescope. If it’s a lousy image, it’s only going to get worse later on.
Without adjustment, the objective lens will produce that image that makes the objects at a specific distance in front of it be sharply in focus at that first focal plane. The objects that are closer will be out of focus and like everything else in optics will be affected by the inverse geometric ratio of the distance from the object to the lens. The closer the object is, the more out of focus it will be and at a progressively faster rate (inverse geometric progression.)
This effect grows in direct proportion with the magnification. Keep that in mind for the rest of this post.
Some scopes will have something known as an Adjustable Objective, or AO. Essentially, this is a mechanism that allows the user to focus the objective lens so your target is in sharp focus at the first focal plane. This is a very simple and efficient way of adjusting the objective and eliminating the issue of parallax. If there are distances engraved on the ring, you will notice the geometric progression such that the further away you focus the lens the smaller the amount of correction needed. Which also means the nearer the object, the larger the correction needed.
Bigger objective lenses cost more money. The bigger the objective the more light that comes in but there is a finite limit to the amount of light that is needed. The tradeoff here is the higher the magnification, the more light you need. A quick way to see what you need is to take the diameter of the objective and divide it by the magnification you want to use. This gives you a number called the size of the exit pupil, essentially the size of the picture that comes out of the ocular lens, what you eye sees. The human eye’s pupil can dilate to a maximum of 6 or 7mm. So anything more than that value is extra, but it allows for very easy setup behind the scope. When the number gets small, like 1 or less, it’s very difficult to get into the exact position to see the image. Let’s take a 3-9X36 scope, something quite mainstream. At low magnification, the exit pupil is 12mm (36/3) and at high magnification it’s 4 (36/9). That’s very acceptable for all magnifications. When you get into magnification factors like 20X or higher, that changes rapidly. Consider a Weaver T-36 which is a fixed power 36X40 scope. The exit pupil is 1.11mm. That’s pretty critical and difficult to see properly, since there’s so little light coming out of it. The light transmittance becomes very important here and that’s a factor of the lens coating (more on this late on.)
So determine the maximum magnification you will want to use and then consider the size of the exit pupil you want to have and remember that the amount of light for that exit pupil will be affected by the overall light transmittance inside the scope.
You want to protect this objective lens and there are a few things to consider. The first one is to use a lens hood or sunshade. Using a lens hood protects the lens from fingers and other disgusting things. It also keep the stray light from the sides to come into the system and create optical artefacts. The higher the quality of the lens the less these artefacts, but when I’m shooting almost into the Sun, the sunshade eliminates that stray light from coming into the scope at all. And if I bump my scope against something, the worse that can happen is I need to replace of fix a broken or bent sunshade and not kilobuck objective lens.
Another way to protect the objective is to use a filter. In fact, all my camera lenses have a UV filter on them, a quality multicoated filter, not a piece of junk. I’ve been playing with filters on riflescopes for a few years now, and I still think that for protection, you can’t beat a sunshade, since filters add yet another lens to a system that is already full of lenses and this is in front of the objective lens, so its effect on the amount of light is out of proportion with its use.
Manufacturers have special coatings for protection of the objective lenses, more on those later.
The downside of a sunshade is that it adds bulk to the rifle. You might consider a smaller sunshade if bulk is a problem. Sunshade dimensions are dependent of the magnification of the riflescope. Very high magnification scopes can have long sunshades that would block the image in a low magnification scope. Buy wisely, and watch that starting magnification.
The focal planes.
This is not a cost factor, just an explanation on how riflescopes work. In a variable riflescope there are three planes where the image of the objective is in focus. The first one is right behind the objective lens but at this point, the image is upside down and inverted right to left. The inner tube contains something know at the erector cell that takes this image and “erects” it for proper viewing and focuses it on the second focal plane. If the scope is a fixed power, the eyepiece or ocular lens, is focused on that plane, the second focal plane. In a variable scope that second plane is focused on the zoom lenses which will then focus the image into the third plane for the eyepiece to focus on.
The lens coatings.
In optics, some of the incoming light is reflected back when there is a change in medium. So for our purposes, every air/glass surface represents a loss of about 5% of the light coming to that surface. When you have just a few lenses, that’s not too bad; 95% at the objective lens, 90.25% after the objective lens, 86.75% in the second lens and 82.45% after the second lens. But you can see how quickly that diminishes with each lens.
Many years ago, camera lens companies started using coats of some material to greatly reduce the amount of light reflected back at the air/glass surface. As you would expect, different wavelengths experience different reflectivity factors. So manufacturers use multiple coats to address the various wavelengths, so that the image that is transmitted keeps the various wavelengths in the same proportion as the objective image, hence the use of the term, multicoated lens.
It is important to coat all the lenses at all air/glass surfaces and this is commonly referred to as fully coated lenses. So, always look for the terms fully multicoated lenses.
Some manufacturers also have additional coating to repel water, dust and other stuff and to make lenses harder, more difficult to scratch. Look for the qualities that you require, remembering that coating costs money but as manufacturers get good at it, the price does come down.
Most riflescope companies do not make their own multicoated glass, they buy it from companies that produce such glass. Companies that make such glass are called Nikkor (Nikon,) Schott, etc.
Protect the integrity of the coatings and learn to take care of your lenses. Use proper supplies to clean them, don’t use your shirt, or harsh chemicals and don’t touch the lenses. Look to camera supplies stores for such items.
As one would expect, in comparable quality glass, the higher the magnification the more expensive the glass. To maintain quality performance for the higher magnifications, the lens has to be bigger and heavier and the grinding takes longer and is more critical. When you look at the specification of a riflescope (or any optical device really,) you will see a number or a combo on the left, followed by an X for power and then a number on the right, which is the diameter of the objective lens in millimeters, eg (4X40 or 4-16X40).
About that number or combo on the left; that represents the magnifying power of the riflescope. If it’s a variable that will be a single number and if it’s a variable, that will be the range of magnification, from low to max, separated by a dash or hyphen, eg (2.5-10 or 4-16 or 6.5-20).
The objective lens does not really magnify the image, it just focuses the objective image at the first focal plane and the internal lenses (zoom and ocular), magnify this image for presentation to the eye.
The zoom ratio.
The zoom is the product of the internal zoom lenses in the inner tube. You can figure out the zoom factor by dividing the maximum magnification by the low magnification. The result will usually be a whole number that is 3 or greater. So in the 4-16X40 for example, the zoom range is 4 (16/4) and the eyepiece magnification is 4X. There are some zoom ranges that are not whole numbers. For instance my Nightforce NXS 12-42X56 has a zoom range of 3.5 (42/12).
As expected, the higher the zoom ratio, the more expensive the scope, everything else being equal. Lots of scopes now have a zoom ration of 4, or 5 or even 6. My March X has a zoom ration of 10 (5-50X56). The reason for the increased cost is the quality of the lenses and the assembly of these lenses; each time you push the limits on a component, the more finicky the assembly of the scope becomes.
In your requirements, you need to weight the incremental cost of a higher zoom ratio; it does provide for greater flexibility but it costs more.
The glass quality.
This is a complex subject and while many people claim they see differences in quality, the vast majority of the time they don’t compare the same things. We talked about light transmission and how this is enhanced or corrected with coatings, but other aspects of quality are more difficult to judge. One is resolution, which is a measure of how much detail a lens is capable of capturing, and contrast which along with edge definition (acutance) create a sharper image. Beyond those qualities, we also have the issue of distortion that may come from a lens or the overall riflescope; barreling, vigneting, etc.
We can talk about this at length and we will, and add some pictures and examples and so on, but put simply, this is where the glass is able to show more details of the target crisply as the magnification increases. A good simple example is to have the lens slightly out of focus and then increase the magnification. You get a bigger image of the details, but they are still vague and not quite sharp. Now imagine if the image was in sharp focus and you increased the magnification.
On another note, light as it goes through glass will focus the various wavelengths at not quite the exact same plane and this is what is called chromatic aberration or color fringing. What this does is make edges in the image to be ever so fuzzy. You can see this on a High Power target when the tan rings are not super sharp in the big aiming black. In the photography world, some lens manufacturers started using fluorite glass elements to would eliminate this “distortion”. The problem with fluorite glass is that it’s expensive to make and it’s a lot more fragile than regular glass. About 30 years ago, Nikon invented ED glass (ED for Extra low Dispersion meaning it keeps all the wavelengths on the same plane,) which came very close to the performance of fluorite glass but without the cost and the fragility. ED glass is more expensive than regular glass, but nowhere near fluorite glass cost.
All my Nikon DSLR lenses feature ED glass elements to reduce or eliminate chromatic aberration. Nikon is not the only one that makes ED glass, other manufacturers have their own versions with their own names, but the concept is the same.
More recently, riflescope makers have introduced ED glass in their offerings and this is a boon to the precision target shooter who is trying to surgically place his or her shots on target. In my case, the sharpness of the rings on that target 3000 feet away is superb in my riflescope. Well until the mirage comes to play, but I also believe that ED glass reduces the effect mirage has on the image of the target in the scope.
I’m not aware of any riflescope with fluorite glass but I have heard that one maker will is working to bring out some models with such glass. I’m not clear on how that will pan out, but I’m sure such a scope would be even more expensive.
As I said earlier, there only needs to be a few ED glass elements in a riflescope to tame chromatic aberration. So in a riflescope with maybe 10 lenses, there would only be one or two that are made of ED glass. (See update on fluorite glass on page 4)
The zoom assembly and the ocular lens.
In a variable behind the erector cell you will find the zoom lenses assembly that we discussed above. I also believe that in a side-focus model, there is an extra lens that is adjusted by that knob on the left. The zoom assembly will enlarge the image in the real second focal plane and focus it in the third focal plane on which the ocular lens of eyepiece is focused. This eyepiece, BTW, provides the primary magnification for the riflescope. In a 4-16X variable, the eyepiece is a 4X and it is multiplied by whatever ratio the zoom lens assembly is set to. All these lenses should be fully multi-coated.
The eyepiece or ocular lens’s only function is to focus on the plane where the reticle is located. The eyepiece is adjustable in diopter by simply twisting a specific ring son the eyepiece, which may need to be unlocked first. The easiest way to adjust the eyepiece is to wind it out completely and then aim the scope at a blank wall or the sky. You take a quick look through the eyepiece and see if the reticle is sharp and uniform in color, usually black and then look away. If the reticle is very blurry, turn the knob a full half turn and take another quick peek and repeat. When the reticle becomes much more defined, only turn the knob in quarter turns until the reticle is very sharp. After I reach that spot, I usually go back a full turn and run the drill again in quarter turns until it’s perfect. I then take a break and do something else for a while and then I come back and run that last bit again, until I’m sure of the setting. Then, if there’s a locking mechanism, I apply it and I never touch this setting again, well until my prescription changes.
You should never, ever use the ocular adjustment to compensate for a bad objective focus. Ever. Never.
Don’t even think about it.
Here again, magnification increases the cost of the unit. Pick the primary magnification that will suit your needs.
The adjustment knobs.
The knobs are used to adjust the direction in which the inner tube looks at the image in the first focal plane that is created by the objective lens. The knob on top adjusts the elevation and the knob on the right adjusts the windage or the horizontal. Of course, opposite these knobs there is some form of spring-like device that pushes the inner back against the knob. There are many types of knobs and while some are just like screws with no measurements, others have clicks that allow you to measure precisely the amount of adjustment you are putting on the inner tube. These measurements are usually in one of three units of measure: IPHY (inches per hundred yards), MOA (minutes of angle) or MIL (milliradians). Further these knobs can go into fractions of these units, such as ½MOA, ¼MOA and 1/8MOA. These fractions are a function of the thread used in the screws. Beyond that, a full revolution of a knob can represent a certain number of these units, like a full revolution is 10 MOA or some other number.
The principal quality here is repeatability. One wants to be sure that when 2.5 MOAs are dialed in, the adjustment is exactly 2.5MOA not 2.70 or 2.41 or some such. Also, if I crank in 21 MOA, shoot some and then crank them back out, the reticle goes back EXACTLY where it was before I started moving. You also don’t want to worry about backlash, meaning when I dial it 2 MOAs, I don’t have to worry that after I take the next shot the adjustment will be set or that I have to dial in 3MOAs and take out 1MOA.
Another more recent feature is a zero stop that will allow you to quickly come back to a known setting after moving around some. The problem is that if you have say 80MOAs or total adjustment, that can represent 8 full revolutions or more, and sometimes people get lost counting the revs. With a zero stop you can quickly get back to a known setting and come back to your required setting from there.
All these qualities and features cost money. There are some surprises however, the Weaver MicroTrac feature is one of the very best adjustment mechanism around and it’s found in very inexpensive scopes. I’ve known far more expensive scopes to be totally brain-dead in their adjustments.
Pick the features that you want and more importantly, match the adjustments of the knobs to the reticle you will use (more on reticles later.) It’s kinda stupid to have IPHY knobs and a mildot reticle.
You also need to decide if you need or want target (exposed) knobs because you will be using those a lot, or if you want covered knobs (set once and forget.) For hunting and SF situations, I would suggest the covered knobs, but for competition, long range and other precision pursuits, adjustable exposed knobs are much better.
The fancier the knobs the more expensive the scope.
The reticle and its location.
When it comes to reticle, the choices are endless; from the simple yet effective crosshair to the fancy-shmansy Horus-type reticle, for the target dot to the bullet drop compensating circles, there are endless gimmicks and tricks out there.
Also, there are two locations for the reticle in a riflescopes and their position is rather important when it comes to their use. So let’s talk first about the position of the reticle and then we can start a discussion on the type of reticle
Everyone here has heard about the misnamed FFP and SFP reticles. FFP is incorrectly called the First Focal Plane reticle, when it is actually in the second focal plane, behind the erector cell, but in front of the zoom assembly. The “FFP” reticle is placed in that plane so that it merges with the image focused by the erector cell to present a parallax-free image at that distance. Because it is in front of the zoom lens assembly, when you twist that zoom lens the image, including the reticle, will grow. The upside is that the reticle adjustments and markings are always in proportion to the objective regardless of the magnification, which is great for ranging and hold-offs on unmarked targets, but can be a bear to surgically place said reticle on target at higher magnifications.
The equally misnamed SFP reticle is place behind the zoom lens assembly at the third focal plane, right in front of the ocular lens. The SFP reticle will remain the same apparent size, regardless of the zoom setting which makes it very usable in high zoom range scopes but useless for ranging and hold-off on unmarked target unless set at the proper magnification, usually the highest one.
The choice of SFP or FFP is very important and is very much dependent on the use of the riflescope. As a rule, FFP are more expensive because I believe the skill level required to set the FFP perfectly is even higher than what is required for the SFP, and is thus an added complication, but what do I know?
Another feature of reticles is illumination and there are various types of those. Quality illumination can easily add another $100 or more to a riflescope. If you know you will never need it, don’t waste the money. I love illumination for specific reasons, age is one of them.
Whatever reticle you get, make sure your knobs match the unit of measure. If you get a Mildot reticle, look at getting a Mildot master to go with it, unless of course, you have extensive training.
The BDC things are neat but are made for a specific bullet launched at a specific velocity. Some manufacturers will have methods to customize this for your own load. I have virtually no experience with that so I won’t talk about it.
Assembling a riflescope requires a high degree of skills, and the more extreme the scope, the higher the required skillset and the time required to assemble it and test it. A 3-9X is much easier and faster to put together than a 5-50X where the tolerances are incredibly small. So a quality scope at the top end will cost a lot more than the same scope at the bottom end and it’s all in the skills of the builder.
You can tell the build quality of the scope by how its various knobs work and the lack of rattle and grittiness in the adjustment. It just feels well-made.
The name on the riflescope.
The brand name on the riflescope carries a price or at least, a premium. The ritzier the name, the higher the premium. It may have the same lenses as another, cheaper brand, but the nameplate costs. Of course, you expect less trouble with a scope bearing a well-known name, but it’s not always the case. Everybody produces less than perfect products at times.
Finally, the warranty. Everybody wants unlimited, no questions asked, transferable warranties. These things cost money and that’s applied to the price of the riflescope somewhere. You just don’t get that for free.
I will be adding and correcting going forward.This message has been edited. Last edited by: NikonUser,
"Wrong does not cease to be wrong because the majority share in it." L.Tolstoy
"A government is just a body of people, usually, notably, ungoverned." Shepherd Book
Definitely Sticky Worthy! Excellent, informative post!
So, after reading the OP and the following discussions that may arise, when someone is looking for a riflescope and wants some guidance, we come to the following questions to guide the choice by building a list of requirements rather than just throw in a recommendation for the riflescope you happen to own.
The first question is always "what is the purpose of the rifle?" This comes right after we are told the exact rifle and caliber that will be graced with this new optical marvel. Hunting and plinking are different endeavors compared to competition or going to war. Hunting hogs at midnight will require something different than shooting targets at high noon on a sunny day.
Next comes anticipated budget. There is no need to suggest kilobuck scopes when the budget is in the low hundreds. And the mantra "buy once, cry once," that's for children. Sorry, real life is different and there's no need to spend thousands when hundreds will be just fine. Also, riflescope technology is rapidly advancing and it's stupid to bleed yourself dry to buy a phenomenal scope that in a few years will be run of the mill. Sorry to burst bubbles and gore sacred cows.
Then it's the usual shooting distance distance and the degree of precision required. Don't come here asking for a Hubble type scope to shoot sub-MOA at 1000 yards with an off-the-rack rifle with an 18 inch barrel and factory seconds ammo.
When we know these basic needs we can then start assembling a list of specifications and finally some recommendations that fit the specs will be forthcoming.
No doubt! Please, para!
Dei. Familia. Patria. Victoria.
Don't back up, don't back down.
Thanks for going to the trouble to write this up. It will be very helpful for anyone who makes the effort to read it.
(And I’m happy to say I learned a few things and there was nothing I disagreed with. I’m always concerned when an expert discusses something I think I know about because I’m afraid I’ll discover I’m wrong.)
|Knows too little |
about too much
TL Davis: “The Second Amendment is special, not because it protects guns, but because its violation signals a government with the intention to oppress its people…”
Remember: After the first one, the rest are free.
Excellent - thanks for taking the time to create this post.
Sigfreund, you're killing me. When did the moniker "expert" apply to me? I'm sure you were talking about someone else, because I'm no expert.
The reason I know a little bit about optics is because of my other lifelong passion, photography. I've been reading magazines and books about photography and lenses for over 45 years so some of it was bound to soak in at some point.
We currently live in a golden age of optics, with new technology coming in at a fast and furious pace. The low price optics of today are optically better than the top end stuff of yesteryear.
I recently received a last few items that belonged to my late father. Among them was a binocular with a fancy name from the 1960s, before lens coating was widespread. I remember those binos and looking through them and being amazed at the clarity. So when they showed up on my doorstep, I was excited to see how they fared compared to modern offerings. I was like I was looking at a black and white picture albeit a clear one. The colors are all washed out and dull; what a difference coatings make.
Anyway, it's fun to compare riflescopes with camera lenses. You can see where riflescopes have greatly improved by using the latest advancements in camera lenses; the lag is definitely getting shorter. There is some divergence in the technologies, the big one being the inner tube with the erector cell and the ocular lens which are not needed in camera lenses. And the reticle technology is also not found in camera lenses. Then again, you don't deal with wide angle lenses, fisheye, flash photography, sensors, settings and of course, post-processing and all sorts of other things.
I'm working on the next post for this thread. I want to talk about field of view and magnification.
Finally, if you want to learn more about spotting scopes, go to birding websites. These folks are insane; they will drop $10,000 on a spotting scope so they can detect the underplumage of a hummingbird at 400 yards.
Thanks for the writeup & discussion. Definitely learned/clarified a few things in the first post.
The Enemy's gate is down.
It may be somewhat entertaining for us lay folk to see the nuanced debate between experts on who qualifies as an expert
From this layman's perspective, the distinction is lost between the level of information you guys have versus what you would call an expert. From where I sit, you guys are experts.
That you guys so freely share your expertise with others -- I'm not sure what that's called.
But I, for one, am grateful.
"Wrong does not cease to be wrong because the majority share in it." L.Tolstoy
"A government is just a body of people, usually, notably, ungoverned." Shepherd Book
I also vote that this should be a sticky.
The only thing I would ask to be added is a guide on how to ID your requirements IOT select the correct scope.
It might be interesting to discuss the meaning of “expert,” but I do know one legal definition that might apply. The Uniform Code of Military Justice defines the term (or did the last time I checked) as someone who knows more about a subject than the average person. That’s obviously pretty broad, but it does mean that I’m an expert on guns and some other subjects, including optics. You, however, are more of an expert on optics than I, and I always try to learn from people who know more than I do, so thanks again.
Well now. I just learned something and I sure do not want to argue with the UCMJ.
But what can I say? Optics fascinate me; cameras, spotting scopes, riflescopes.
BTW, I deviated from the pure FOV/magnification thing in my next post and I'm adding more stuff to answer CD228's request. Look for it soon.
Do not meddle in the affairs of wizards, for thou art crunchy and taste good with catsup.
Just DONt go reading and learning about Chromatic Aberration....
it will ruin optics for you lol.
Here's the next instalment.
A riflescope presents an image to your eye that is created by the ocular lens focusing on a portion of the image that comes into the objective lens.
Field of view (FOV) is the capability of a riflescope to present to your eye an image that covers an angular area. As we all know, the higher the magnification, the smaller the FOV. So let’s talk about that relationship and how FOV may be important or not.
Regardless of the magnification of your riflescope, the same picture of the objective is created at the first real focal point. The inner tube is adjusted by the knobs to “look” at a specific portion of this image. So the maximum FOV of a riflescope is really delimited by the inner tube. Beyond that the FOV is a function of the focal length of the optics (or the magnification for us with riflescopes.)
Let’s take that Nikon Monarch 5 4-20X50 that was the object of a recent thread. The manual says the FOV at 100 yards is 20feet to 4.1feet. So that means the inner tube will have an FOV of 3.82 degrees at 100 yards at 4X and 47 MOA at 20X, something like ¾ of a degree. Since the adjustment range is 35MOA, we can infer that the usable FOV at the first focal plane is at least a half degree more so 4.3 degrees.
In other words, at the lowest setting (4X), this riflescope presents a view to the eye that encompasses just under 4 degrees. At 4X, the target at 100 yards appears to be 25 yards away, which is a good thing for placing the reticle more precisely on the target, but it also cuts out about 98% of what you see with your eyes (let’s say you can see 180 degrees.) When you get to higher magnifications that 98% climbs rapidly to be very close to 100%.
These FOV figures will be pretty similar for all riflescopes with equivalent magnification, it’s just a fact of optic life. For most people the FOV is not really an issue; we shoot at targets that do not move and we can take our time to aim. But sometimes, speed of target acquisition is critical and so FOV comes into play. This is where a variable is very useful because with just a twist of the knob you can go from a 4X which makes the 100 yard target appear to be 25 yards away to 20X which make the target appear to be 5 yards away. Except that in your riflescope you only see about 4 feet across at 100 yards; talk about tunnel vision. Oh, and things got a little darker in the scope, but not so much that it’s any issue during daytime.
When people start thinking about buying a riflescope, the first question should always be “what will the rifle be used for?” Hunting and SD will emphasize the FOV aspect, while target and competition will be more concerned with magnification. I’m grossly generalizing here, so adjust as needed. If your requirements are many, you want to be able to hunt in the brush, but also want to dabble at long range shooting or varmint hunting and if your rifle/caliber lends itself to that, your main requirement will be for a scope that has a good zoom range. The regular 3X is kind of limiting, whereas 4 and even more so, 5X is really something that should be at the top of your list of requirements. That will be more expensive than 3X or 4X.
Next you probably want to pick the low end, the magnification at minimum zoom. The difference between 1X and 2X is a 100% increase, going from 2X to 3X is a 50% increase, going from 3X to 4X is a 33% increase and so on, as you see, diminishing returns. On the other hand the high-end magnification increments by 5X for every 1X at the low end and that’s something. So at the 3X low end, we have 15X on the high end, compare to say 4X low and 20X high. But hey, guess what? That’s still only a 33% increase, same as the low end, of course.
On the other hand, if you had your heart set on wanting 20X at the top end that pretty much dictates the low end being 4X. I will tell you that hunting with a 4X scope was very widespread for a long time before the variables became good enough and cheap enough to supplant the fixed powers.
So it’s up to you to pick either the low end or the top end as the main requirement and perhaps you do not need an expensive wide zoom range, but it’s nice to know that’s available.
So, how does one decide the top end magnification required? I always ask how precise do you have to be and at what distance. The unstated question is of course, can you even hit the target of that dimension at that distance?
So let’s say that you want to hit a 3 inch target at 600 yards; that’s your requirement; half MOA to 600. It’s pretty difficult to hit something you can’t see so the first order of business is to figure out what kind of magnification you need to be able to discern that target. I’ve been told that normal visual acuity AKA, 20/20 vision, is being able to distinguish a 2MOA letter separated by a 1MOA space between the letters. Because MOA is a measure of angular area, the difference between 2MOA and 0.5MOA is not ¼ but 1/16 because there’s a square in the formula for calculating the size of the area.
So, right off the bat for a normal person, the target size is 1/16 the size of what this person can discern. I know that when I’m at the range at the 600 yard line, when the target comes up I can see the aiming black just fine, it’s 6MOAs. There is no way that I can see the shot spotter which is .5MOA on the target and white on black. I can barely discern that a scoring disk may be on the target and that’s 1MOA; I would say that 2MOA is pretty much correct for my corrected eyes, 20/20. I used to shoot at 600 yards with a 10X scope. I could not reliably make out the .5 MOA inner ring, but since the rings where concentric, hitting the middle .5MOA ring consisted of aiming for the middle of the 6MOA aiming black. It’s not that I could SEE the .5MOA ring, but I knew where it was supposed to be. That’s not the same thing.
When I moved to a 20X scope, I could pretty much make out the X-ring but that scope had a think reticle that essentially covered the X-ring. So 20X should be adequate for the purpose. Barely. At 100 yards, it you can pick out the exact square inch block on the target that you want to shoot, you’re still only at 1MOA. I can probably identify a 4 square block (2MOA), in no way can I spot a quarter of a square (.5MOA).
When next you’re at the range, look at the targets and see what you can discern and reliably aim at, and with what magnification.
did you get this from someplace ?
is there a link ? that you could offer.
Safety, Situational Awareness and proficiency.
Neck Ties, Hats and ammo brass, Never ,ever touch'em w/o asking first
I have no doubt that this information is something that NikonUser is fully qualified to present and discuss himself.
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