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Riflescope primer now with Hyperfocal Distance & other optical properties 7/15/2021 Login/Join 
Freethinker
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Thanks for the explanation of the eyepieces. I believe I am actually starting to understand some of this stuff. Smile

(Because the topic is pinned, I often don't notice new posts immediately.)




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 44345 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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As I have explained many times earlier, I am a huge fan of March scopes. I believe them to be among the best is terms of optics (glass, zoom, eyepiece, etc) and incredibly robust in construction. The attention to detail is astounding.

That said, I have been informed that March is introducing an LPVO (Low Power Variable Optic) with some leading-edge features:

It is a 1-10X24 (10X zoom) and it features a Dual Reticle. This is not something new, but to have such a reticle from March means that they are confident in their design and manufacturing.

At least confident enough to produce a few for testing. Let's talk about reticles.

We are all familiar with the two main types: FFP and SFP. The FFP (First or Front Focal plane) reticle is located at the front of the erector tube and this is where the image from the objective lens group focuses the image. By placing the reticle in the focal plane in front of the erector zoom lenses, it means that the formed image including the reticle, will increase in apparent size as you zoom in. This has the benefit of keeping the reticle in constant relationship with the target image. The subtensions on the reticle remain the same regardless of the zoom setting. The downside is that your reticle will grow in size and will cover more and more of the image as you zoom. When you're dealing with the mundane 3X or 4X zoom ratios, that's not really an issue, but when you have 8X or 10X ratios, the reticle designers have to get more innovative to produce reticles that are useful throughout the zoom range.

The SFP reticle is located at the back end of the erector tube assembly and that's the image that the eyepiece presents to your Mark 1 eyeball. The image of the target can grow and shrink as you zoom in and out, but the reticle stays the same size at all times. This means that the subtensions on the SFP reticle are only easily meaningful at certain zoom settings. For instance, in my March-X 10-60X56, the MTR-5 reticle represents exact distances when it's set at 40X. When I boost to 50X, the subtensions are different by 25%.

In high magnification target scopes, the SFP reticle is more useful as you keep the same size reticle and you usually dial to make adjustments. The SFP reticle can be a lot finer and much more precise for that kind of application. For instance, no one uses an FFP scope in F-class where the minimum magnification is usually north of 30X and the distances and target sizes are well known.

In PRS-type competitions where surgical precision is not required but where targets are at varying distances and can be of different sizes and shape, an FFP design is eminently more useful, almost de rigueur. I doubt anyone uses the FFP reticle for ranging, but knowing that your subtensions are the same regardless of magnification allows you to correctly hold much more quickly than if you had to deal with an SFP reticle.

Now we come to the Dual Reticle. The concept is to have the FFP reticle set up as a scale with the subtensions that are useful for aiming, something like a line at every 0.1MIL and a thicker one at say 1.0MIL and/or 0.5MIL. Something along those lines. The SFP reticle can be just a simple crosshair with maybe a center dot that can be illuminated. This crosshair will stay the same size regardless of the zoom setting, but the scaled FFP will grow or shrink and will always have the same subtension values at whatever zoom setting. It will appear to grow and shrink in the eyepiece and the crosshair will remain the same size.

This gives you the best of both worlds, especially in a 10X zoom.

The following thoughts are mine and mine only. They do not represent March or anyone else's thoughts, concepts, designs or challenges. If I am wrong, and that's highly likely, it's my own fault.

Let's talk about the manufacturing challenges as I would expect them. In order to make this dual reticle work, the erector tube assembly will have to be near perfect. (I was going to say perfect, but I don't think there's such a thing.) The front and rear reticles will have to be perfectly (there's that concept again) aligned. But wait, there's more. Even if they are "perfectly aligned" the erector tube has moving parts; the zoom lenses. It is well-know that zoom scopes can display POA shifts when the zoom ratio changes. This is due to having zoom lenses that are not (oh no, not that word again) perfect. For example, I had a Nightforce NXS 12-42X56, a superb scope, that exhibited a 1.5 inch displacement when I zoomed from 42 down to 12, shooting at 50 yards with a .22LR. It was uncanny; it did it all the time and exactly the same amount and direction, every time I shifted the zoom setting from 12 to 42 and back.

So erector tube assembly: tube, reticles, zoom lenses: will have to be (once more, with feeling this time) "perfect" or as perfect as possible. If any scope manufacturer can design and produce such a scope, 10X zoom dual reticle, I can think of no other outfit than March. I am looking forward to find out how it comes out; it's supposed to go to early testing shortly and if it works out, it will be available by year's end. I wish them well.

You can read more about the new designs they have going to early testing here: https://marchscopes.com/news/5637/

The astute reader will see the mention of the 4.5-28X52 with the FML-PDK and FML-LDK reticles. Yep, those are the ones that offgrid and I put together for them.
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
Freethinker
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Thanks for the heads-up in the other thread; I probably wouldn’t have noticed this for a bit.

I am confused by the “dual” reticle. Are both reticles visible through the scope at the same time? That seems unlikely, but I’m not picturing how they work.

And to quibble about one statement of your typically excellent discussion in reference to a first focal plane reticle:
“The downside is that your reticle will grow in size and will cover more and more of the image as you zoom.”

Because the image also grows in size as we zoom up in magnification, the amount of the image covered by the reticle does not change. If I can see the small central diamond of the targets I like to use behind the reticle at one magnification setting, I can see it at all settings because it changes apparent size along with the reticle. (If I misinterpreted your statement, my apology, but that’s how I understood it.)

I would say that the problem with the FFP reticle is that because it shrinks in apparent size as the magnification is lowered, it must be thick enough to see at minimum magnification. And therefore at high power it’s a lot thicker than it probably would be in a SFP scope. FWIW this hard-to-see-at-low-power and thick at high issue is why one law enforcement sniping authority recommends SFP scopes for that purpose. Not everyone agrees with him, but his point is easy to understand.




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 44345 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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The dual reticle is exactly as the name says, you see both reticles in the scope. One superimposed over the other. So the SFP shows the crosshair at the exact same size throughout the zoom range. And while you are zooming, you actually see the FFP reticle grow in size on the crosshairs.

What you see is an actual SFP reticle that seems to grow lines as you zoom in.

You're right about my choice of words. What I meant to say is that what you see as a small reticle on a target will grow in size in relation to the size of the whole image, not the target itself. And for those of us who are used to fine reticles staying the same size as the target grown in the scope, we can be more surgical in placement, something that becomes difficult as the FFP reticle grows also.

While working on the FML-PDK, I was struck by how much the reticle grows in an 6.2X zoom. To the point where the reticle really takes over the entire image. (That's why we worked so hard to make it "airy.") I can only imagine how big a 10X zoom would grow the reticle.

Now, as an F-class shooter, I have to be able to place my reticle, for example, on the left lower quadrant of the X-ring, which measures 0.5MOA. My non-expanding SFP reticle allows me to place the 0.125 MOA dot at 40X exactly where I want it in the X-ring. If I want it finer, I can increase to 50X and that dot now subtends 0.1MOA. I would hate to see what a 0.125MOA dot would look like at 4X in an FFP design.

I hope this makes sense and thanks for drawing attention to it.
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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You know, sigfreund you ask good questions.

Following up on my answer to your question just above, it occurs to me (I know, I'm old and slow) that the DR-1 reticle will benefit from the increased precision of the SFP crosshair growing finer in relation to the target along with the consistent ranging and subtensions of the FFP scale as you go up the zoom range. That's probably a better way to look at it.
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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Here is a link that provides a lot more details about the dual reticles.

https://marchscopes.com/news/5780/
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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As I have previously reported, I have been testing a March-FX 4.5-28X52 HM WA for the last few weeks. I am not at liberty to go into any kind of detail about it at this time (NDA) but I have been taking pictures through it and comparing it to other riflescopes I use. I sent the below picture to March over the weekend and they liked it very much and said I could post it online.

The picture shows the difference between a regular eyepiece (around 20 degrees of coverage) and the wide angle eyepiece found on the new March-FX 45-28X52, which is 25degrees (ie. 25% more.)

The two riflescopes were set up on tripods and placed side by side at about the same elevation. The one on the left is a March-X 10-60X56 HM, a riflescope with which I am extremely familiar as that is the one that sits atop my Match FTR bolt action rifle. The one on the right is the March-FX 4.5-28X52 HM WA with the FML-PDK reticle. I set them at 10X, which is the minimum magnification for the 10-60X56 and adjusted the March-FX also to 10X, by comparing the apparent size of the number 20 on the target. I did not simply rely on the engraving on the zoom ring, I tried to make sure that I had the same magnification by looking. Indeed the 10X engraving was virtually dead on, no real surprise there.

Next came the difficult part, taking pictures with a smartphone through a riflescope. It took about a half dozen attempts each to get a decent through-the-riflescope picture for each scope.

You can see that the eyepiece for the March-FX is substantially bigger than the eyepiece of the March-X. That is how you get a wider field of view and still maintain the magnification. There is no other way; the diameter of the eyepiece has to be bigger to get a bigger FOV. It really is like the difference between a movie in IMAX vs one in regular format.

You can see the FML-PDK reticle in eyepiece on the right. I also added a white circle that shows the FOV of the regular eyepiece on the expanded image of the March-FX. This white circle is not part of the reticle.



You can view it in greater detail here:
http://img.gg/vH9awOI

This message has been edited. Last edited by: NikonUser,
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
Freethinker
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Fascinating stuff. Thanks.

Is it my imagination, or is the resolution of the -X slightly better than that of the -FX?
It’s difficult, of course, to be sure with a small Internet photo even at the link, but I’m basing that on the vertical and horizontal bars of the chart immediately to the left of the “10” block.




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 44345 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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These pictures are not truly representative of the actual resolution capabilities of these riflescopes. I'm actually just happy that some of them came out decent enough to show the FOV difference. This is handheld stuff, with me getting frustrated trying to get some results. I started with the March-X as baseline and then did the same on the March-FX. By that time, I was shaking a little bit and getting hot and bothered. (Getting old is a b*tch.)
 
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Freethinker
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All right; I appreciate that and will admit they are actually exceptional for handheld efforts. Smile




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 44345 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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I have been testing one of those March 1-10X24 with the Dual Reticle. I did a quick photo session with my smartphone to show the reticle. Here is a link to my Nikon Image Space with a video and some photos.

http://img.gg/wnMFjN2

The video shows the reticle going through the magnification range up and down. It's a little fuzzy, but it shows the concept The photos show the setup on my tripod and then my least terrible pictures through the scope at various magnifications from 1X to 10X. You can see the magnification fast lever going up on the left side as the magnification increases.
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
Freethinker
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A nice demonstration. Thanks!




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 44345 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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Great thread and excellent info.

I was just looking at the Shmidt and Bender 1-8x24 dual reticle scope the other day.
Nearly $5K for that one. Suffice it to say I'm not in any danger of getting one.

I have a Primary Arms 1-8x24 on my AR10 but should have went bigger to begin with.
I came across a Vortex 6-24x50 Diamondback MOA reticle for a good price and picked it up.
Might be a bit more than I need but I figure the bigger objective would be better suited for overcast skies.

With all of the different scopes available these days this thread helped me sort out all the various features.
Quite a lot to choose from in the $500 range.
 
Posts: 815 | Location: Portland Oregon | Registered: October 01, 2011Reply With QuoteReport This Post
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Thank you for the nice comments.

Yeah, the S&B offering you are talking about is less magnification, larger, heavier, and with an FFP, a lit dot instead of a full dual reticle and much more money that the March 1-10X24 with dual reticle. What's not to like?

I think I did mention that it was optically extremely difficult to have an objective larger than 28~mm in a 1X scope. If you want a bigger objective, you need a a higher magnification. That's a reason I am so impressed with the March 1.5-15X42. They had to come up with a whole new design for the objective lens group to have a 42mm objective in a 1.5X scope
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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This thread is more of a "knowledge thread" and the discussion is more focused (pun intended) on the mechanics and technology in a riflescope as opposed to just using them and selecting one versus another. I'm going to talk about a subject that I covered a little bit on another forum.

So, without further ado, let me introduce the concept of Hyperfocal Distance.

Hyperfocal distance is an optics term that refers to the distance at which a lens is focused to the point where the lens will provide the greatest depth of field. To make it even simpler, it's the MINIMUM distance at which the objects at infinity are acceptably sharp.

The formula for calculating the hyperfocal distance of a lens is:

H = L x L / F / CoC / 1000, where:

H is hyperfocal distance in meters.
L is the focal length of the lens.
F is the F-Number of the lens.
CoC is the diameter of the Circle of Confusion in mm.

As we have discussed many times in the past, a riflescope is essentially three parts: the objective lens group, the erector assembly, and the eyepiece. The magnification range of a riflescope is calculated by dividing the focal length of the objective group by the focal length of the eyepiece and then multiplying by the zoom ratio of the erector set.

Riflescope makers do not divulge the focal length of their riflescopes; most consider this a trade secret as part of their design. Without the focal length, we cannot use the hyper focal distance formula. However, we can guestimate the focal length and work from there. I will explain how to do that in a subsequent post.

But before we get to that, let me explain the benefit of knowing the hyperfocal distance of your riflescope. In fact, just knowing the focal length of your scope will allow you to determine focus distances that will provide you with the greatest DOF for your application. This will be most useful in scenarios where you get to shoot over a wide range of distances in a short time and you don’t want to be fumbling with the side focus knob. I will provide examples and calculators.
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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Now for the hands-on portion of the exercise: measuring the focal length of the objective lens group. This is the important part because, as we have already discussed earlier in this thread, the objective lens group forms the first image. The rest of the riflescope depends on that first image, there is nothing you can do optically to improve it. The mirage distortion, the focus, the DOF, etc, that's all here. After that the image is magnified and the less perfect it is, the more the flaws show up.

The focal length of this portion is the distance between the objective lens and the FFP. We have to make some assumptions, here because we can't really open up the riflescope to make sure where everything is located exactly.

The objective lens is composed of more than one lens; it's usually a doublet, a pair of lenses close together. In a regular riflescope the objective lens housing is larger than the main tube and there is a distinct place where the taper from that larger housing to the main tube diameter begins. This is also where the back of the objective lenses is located. The beginning of the focal length, will be located in front of the beginning of the taper, these lenses are thick. So I usually allow about a half inch in front of the start of the taper. This is all (informed) guess work, but we are not looking for ultimate precision here.

The FFP, where the first image is formed, the one from the objective lens, is located at the front of the erector tube. The erector tube is the one that is moved to adjust the zero for the riflescope and the area of greatest movement will be at the adjustment knob. Therefore I use the middle of the adjustment knob as the location of the FFP. You then measure the distance from the area we located on the objective lens housing to the middle of the adjustment knob, but doing it parallel to the main tube, not at an angle. Do it a few times and you will get a pretty good idea of the focal length of the objective lens group. I'll finagle a graphic in a bit and attach it here.

Once you have this value, you have the key for all the calculations you want to do.

As a note. The LPVOs are a little bit of a special case because they usually do not have a bigger housing for the objective lens. What you need to do is see where the front objective lens is and then visualize where the second lens is going to be, realizing that these lenses are not very thick. (Remember how you calculate the magnification of a riflescope and the base magnification in this case is 1X or near to it. So the front objective group will have the same focal length as the eyepiece group; it's the erector that does the magnification. That's also why you don't have a large objective lens when you're going into a 30mm tube.)

Let's say that your "measured" focal length is something like 80mm. You know that your objective lens diameter is 42mm. You can now calculate the F-number of the objective lens group: F= L / D where F is the F-number, a dimensionless value; L is the focal length in mm; and D is the diameter of the objective lens, also in mm. So, for a 42mm objective with an 80mm focal length, the F-number is 80/42 or 1.9. The F-number is 1.9 of written another way, f/1.9. In photographic terms, that's a pretty fast lens.

Now you can already see that you can use the formula I posted earlier to figure out the hyperfocal distance. For now, use .030mm as the value for the Circle of Confusion. I'll explain more about the CoC in the next post.

This message has been edited. Last edited by: NikonUser,
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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quote:
Originally posted by NikonUser:

In the meantime, I acquired another March scope, this time a March-X 10-60X56HM, which is very similar to my existing 5-50X56 but this one has Super-ED glass for even better CA control. I have assembled it in its rings and mounted it on a rail. I hope to sight it in on my match F-TR rifle this weekend. I’ll post some pictures soon.



Did you go with the MTR-5 reticle in this new scope?


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"Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it."
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Posts: 9705 | Registered: January 17, 2011Reply With QuoteReport This Post
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quote:
Originally posted by wcb6092:
quote:
Originally posted by NikonUser:

In the meantime, I acquired another March scope, this time a March-X 10-60X56HM, which is very similar to my existing 5-50X56 but this one has Super-ED glass for even better CA control. I have assembled it in its rings and mounted it on a rail. I hope to sight it in on my match F-TR rifle this weekend. I’ll post some pictures soon.



Did you go with the MTR-5 reticle in this new scope?

This was quite a while back. Yes, my March-X 10-60X56 HM also has an MTR-5 reticle, which I consider to be the best F-class reticle currently, for old decrepit eyes. The MTR-2 is the best for younger eyes.

This message has been edited. Last edited by: NikonUser,
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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The Circle Of Confusion (CoC).

This measurement is used as a focus criteria. Look it up, it is a real concept in photography, and as I found out, in riflescope optics (and probably all manners of optics, but let's stay on riflescopes.)


I copied this from a site called studiobinder.com as it explains the CoC in ordinary terms, better than I can.

"The circle of confusion is the measurement of where a point of light grows to a circle you can see in the final image. Also called the zone of confusion, it’s measured in fractions of a millimeter. The circle of confusion is what defines what’s in or out of focus. This number is also what calculates depth of field. The circle’s size is what affects the sharpness of an image. The smaller the circle, the sharper the image. And the larger the circle, the blurrier. It is often written as CoC."

So the larger the CoC, the blurrier the image. The diameter value used by some riflescope makers as the upper limit for "acceptable focus" in the DOF is 0.030mm. I can assure you that as you near the exact focus distance in the image, the CoC value shrinks rapidly. But for purposes of calculating the range of the acceptable DOF when the focus is set at specific distances, we shall use 0.030mm. I would also suggest that if you spend your time at the upper end of a high zoom ratio riflescope, you might want to reduce the CoC value in your calculations, maybe close to 0.025 or 0.020mm.

Next up will be some examples with a specific riflescope. You can, of course, substitute your own riflescope, now that you know how to calculate the focal length of the objective lens group and you already have the diameter of the objective lens.
 
Posts: 3300 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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I'm going to use the March-FX 4.5-28X52 as an example of a quality riflescope that would be used in fast moving, varying distances scenarios. It's an FFP design with Super-ED glass.

The 4.5-28X52 has a 6.22X zoom range. Its objective focal length is about 115mm by my crude measurement technique described above and which I just performed. If the F-length is 115 and the objective lens is 52mm, the F-number is about 2.2. It's a pretty "fast lens" in photography terms (any telephoto with an F-number of 2.8 or less is fast). This has advantages and disadvantages and we will look at both so you can apply it to your riflescopes and perhaps even consider these factors in future purchases. Before going any further, let me introduce a great on-line DOF (Depth Of Field) calculator: dofmaster.com.

You will see that it has 4 data input: CoC, F-length, F-number or F-stop, and subject distance with the unit of measure. We know the F-length and the F-number. To calculate to hyper-focal distance, we do not need the subject distance, but we need a value for the CoC. As I explained earlier, I know that some high-end riflescope manufacturers use 0.030mm as a focus criteria for the DOF. The smaller the value the higher the quality of the picture and the more reduced your acceptable DOF is. The reverse is true.

Using a CoC or 0.030mm, the DOF calculator shows us that the hyperfocal distance is 196 meters. This means that if we focus the 4.5-28X52 perfectly on a subject 196 meters away, the DOF will go from 98 meters to infinity. The focus criteria, CoC, will not exceed 0.030mm throughout this DOF. Put another way, this is an acceptable sharpness of focus for the entire DOF, whereby we can aim and properly on any target in that DOF without touching the side focus knob.

Right now we know that is we focus at 196 meters, we have the greatest DOF for that riflescope running from 98 meters to infinity. We could work out another setting that would give us good coverage from near to about 100 meters. It turns out that a focus setting at 67 meters will give us an acceptable DOF from 50 to 101 meters.
Therefore, if I played a game where the targets got from 50 meters out, I could have two settings marked on the side focus, one for near and the other one for anything past 100 meters. This way you could set the focus even before you got into position behind the riflescope. Of course, this does not mean that you can’t tweak the focus on the far targets, especially if it’s a tricky target, but the DOF limit of 0.030 is pretty sharp.

Next up we will look at other scopes and look at advantages and disadvantages of different diameter objective lenses.
 
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