Before looking at other riflescopes, I wanted to bring up a point, which I think is important; how can we change the parameters of a given riflescope.

In our current example, we have the focal length at 115mm. We can't change that. We have the objective diameter at 52mm, but that we can change. March supplies a modifier disk with this (and some other) riflescope. This is cap that screws in at the front and it has a hole in it. In this case, the modified disk is 35mm in diameter. This represents a 50% reduction in incoming light, which provides a boost of 1 F-stop to 3.3. So by adding that modifier disk, we increase the F-number. I will also say that a drop of 50% of light is not significant in daylight, unless it's really cloudy and late or early in the day.

Let's see what that does to the hyperfocal distance. We will use f/3.2 because there is no f/3.3 in the calculator. This is a photography calculator and there is no f/3.3 lens.
The hyperfocal distance is now 139 meters and ranges from 69 meters to infinity. What about a near distance DOF? With the focus at 47 meters, we go from 35 meters to 71 meters. So those two distances could be our setup to easily cover from 35 meters to infinity without even looking through the scope.

As we can see the diameter of the objective lens has a big influence on the DOF. Let's keep that in mind when comparing riflescopes. A wider objective will give us more light when that is needed (higher magnification, longer shooting time, and also better resolution when using the same level of glass), whereas a smaller objective will provide a deeper DOF, which some people think makes for a clearer picture, but actually provides for less resolution.

This message has been edited. Last edited by: NikonUser,

Let's examine another scope, this time a March-FX 5-40X56. I measured the objective focal length at about 180mm. The objective diameter is 56 so the F-number is 3.1, which is the same as the 4.5-28X52 with the modifier disk.

However, even though the F-number is the same at 3.2, the focal lengths are different, 115 Vs 180. Let's see what that does to the hyperfocal distance.

For the 5-40X56, the HD (hyperfocal distance) is 340 meters, from 170 meters to infinity. That is radically different compared to the 139 meters, from 69 meters to infinity of the 4.5-28X52 with the MD.

Of course, there is also a modifier disk available for the 56mm objective and it does the same as the other one and will add a full f-stop to the number, bringing it to 4.2. Unfortunately the DOFmaster does not have an f/4.2. Only a f/4 and f/4.5. We can calculate for 45, but I'll use f/4.5 for now.

With the MD on that 5-40X56, the hyperfocal distance is now 240 meters from 120 meters to infinity.

Very interesting and informative discussion, NikonUser. Thank you as always.

Let me ask to ensure I understand: The CoC isn’t something that is a measure of lens quality but rather is used to describe image quality; i.e., how far out of perfect focus the image can be and still be acceptably sharp—correct?

You mention sometimes using a CoC of 0.02 or 0.025mm at higher magnifications. Why would it be smaller at higher magnifications?

Added: And I assume that the depth of field calculation/acceptability has nothing to do with parallax—? I.e., to use your one example of sharpest focus at 196 meters and acceptable focus from 98 meters to infinity, that doesn’t mean that if the focus were set at 196 meters that the image would be (necessarily) parallax free from 98 meters on; is that correct?

quote:

Originally posted by sigfreund:
Very interesting and informative discussion, NikonUser. Thank you as always.

Let me ask to ensure I understand: The CoC isn’t something that is a measure of lens quality but rather is used to describe image quality; i.e., how far out of perfect focus the image can be and still be acceptably sharp—correct?

You mention sometimes using a CoC of 0.02 or 0.025mm at higher magnifications. Why would it be smaller at higher magnifications?

Added: And I assume that the depth of field calculation/acceptability has nothing to do with parallax—? I.e., to use your one example of sharpest focus at 196 meters and acceptable focus from 98 meters to infinity, that doesn’t mean that if the focus were set at 196 meters that the image would be (necessarily) parallax free from 98 meters on; is that correct?

A pair of good questions, sigfreund.

1- Yes, the CoC is a measure of the quality of the focus for which the lens is capable. I do not profess to know anything about how lenses are assembled, but once I saw the measurements of specific lens placement, my little mind was blown. They were taking about hundredths of millimeters, if you can believe that. So to my mind, the sharpness of the focus is a product of quality glass for sure, but even more importantly, quality of the assembly. This is why it takes so long to build these high-quality riflescopes.

2- An excellent question. I take you back to how a riflescope works: The objective lens group captures the image of the objective and focuses it on the FFP. This is the focus we are talking about here. The quality of the image (IQ) at the FFP is critical for the rest of the riflescope because it will not be improved further down. Now the second part of the riflescope is the erector assembly with the zoom lenses This assembly will flip the image around and will start to magnify it. In fact, that's the mechanism that takes the image provided by the objective lens and then can zoom in 6.22 times. This is like looking at a picture that you took with your digital camera and zooming in on the screen. Any defect in the picture will rapidly show up and the final picture will look crappy. This picture is formed at the SFP. Then you have the magnification provided by the eyepiece itself. It's like a 5X loupe or some such.

So to my mind, when you are using the low to mid range of the zoom, a CoC of .030mm at the FFP is fine, but when you get to 5X or so, you might want to be more critical of the image at the FFP and reduce the CoC for your DOF to keep the IQ of the final image going to your eye.

All right, now I believe I understand what I was asking about. Thanks again for taking the time to explain further.

On a totally different area of optics, Deon has just posted an article at their site that should be of interest for those who shoot at long distances and have to contend with mirage or shimmer.

https://marchscopes.com/news/12385/

It has to do with the type of glass used in a riflescope and how ED and especially Super ED glass has been found to have unexpected qualities.

A few years ago, I realized that on heavy mirage days, contrary to all others on the F-class firing line, I was not lowering the magnification of my riflescope from 40X. This was not always the case; when I had a NF NXS 12-42X56, I would sometimes drop to the mid-20s are low 30s on days of mirage. Prior to that, my Weaver T-36 was completely unusable in heavy mirage days. But shooting with my March-X 5-50X56 allowed me to stay at 40X all the time, year-round. I finally put 2 and 2, together and postulated that the ED glass in the March was somehow resistant to mirage. Don't get me wrong, the IQ was affected by mirage, but the target kept its shape, I could still see the rings and I could place my central dot surgically on the target.

A few years back, I got a March-X 10-60X56 HM which sports Super ED glass. (I've been meaning to discuss glass types in my stickied thread, just been too busy.) I now shoot at 50X all the time, year-round. The Super ED glass is much more resistant to mirage than the ED glass. I did report this to Deon, the makers of the March riflescopes and the pioneer in ED glass riflescopes, and the only ones who use Super ED glass. They dismissed my observations as simply impossible; glass doesn't do that. After I started talking about this everywhere, other shooters noticed this and reported to Deon. One day, the glass designer at Deon went out and tested specifically for this. And discovered that I was not delusional. Deon reported on it at their website and it is now accepted that Super ED glass in a proper riflescope design has what they call "shimmer protection" capabilities. "Mirage" is a misnomer and they rightly term it "shimmer."

This effect cannot be achieved with traditional glass, regardless of how excellent it is. The ultimate Extra or Ultra low Dispersion (ED) glass is made from pure fluorite crystals, CaF2. There is only one company that makes spotting scopes with CaF2 glass and that is Kowa. They have 3 models with that glass: the 880, the 99x and the High Lander binoculars. I recently bought a Kowa 883 precisely for the CaF2 glass. I did not even consider any other spotting scope as none of them can match the shimmer protection of pure fluorite crystal glass, regardless of price. The Kowa 99x was too big for my needs.

The CaF2 glass allows you to discern the waves of the "mirage", you see the river but it doesn't mess up your image anywhere near what happens in traditional optical glass. It is impossible for traditional glass to match the IQ of a CaF2 glassed spotter.

At your suggestion, I’ve repeated the relevant part of my post in the other thread here:
------------------

I believe it is what you meant, but to be clear, no glass of lower quality than “Super ED” will offer improved seeing through heat wave effects (“shimmer”)—correct? And in your experience is there any minimum “badness” for the advantage to become apparent; e.g., minimum range to the target? (I keep thinking I should take all my rifles to the range some hot, sunny day and compare how the scopes deal with heat waves. I have only a couple of hundred yards, though.)

What I would be interested in are actual descriptions of differences in difficult seeing situations. I watch many NRL Hunter series videos and the most common comment by competitors is how hard it is to find its often small, cryptic targets. No one ever says, though, “I did much better this time at finding targets since I switched from that crappy Tangent Theta to the SuperClear 0.5-72×88mm scope.” Why is that? Because, as you alluded, at a certain quality level (short of the Super ED) the differences are so small as to be indistinguishable—? Or do the optical differences matter for only certain things such as dealing with shimmer? Thoughts?

quote:

As always, NikonUser, thanks for that informative discussion.
I believe it is what you meant, but to be clear, no glass of lower quality than “Super ED” will offer improved seeing through heat wave effects (“shimmer”)—correct? And in your experience is there any minimum “badness” for the advantage to become apparent; e.g., minimum range to the target? (I keep thinking I should take all my rifles to the range some hot, sunny day and compare how the scopes deal with heat waves. I have only a couple of hundred yards, though.)

I have made the comment about not being able to see through heavy haze or “mirage” effects in the past, but it is of course possible to see something through almost any atmospheric condition short of dense fog or clouds. I should probably just say that the view though any optical device I’m familiar with can be severely degraded by certain atmospheric conditions, and some of the worst I’ve had to deal with were on long distance rifle ranges.

Something I have seen very little mention of in discussions about shooting optics is what practical effects different levels of quality have. I believe it was DLO who observed that for almost all situations all that really matters is whether the shooter can see the target. That should probably be caveated with, “… see the target well enough for an accurate sight picture.” Even then, though, it does usually mean that in practice a bit of color fringing or even blurring at the edges of the image that would usually result in a poor review is unlikely to mean that we couldn’t use the scope to hit a clearly visible target.

What I would be interested in are actual descriptions of differences in difficult seeing situations. I watch many NRL Hunter series videos and the most common comment by competitors is how hard it is to find its often small, cryptic targets. No one ever says, though, “I did much better this time at finding targets since I switched from that crappy Tangent Theta to the SuperClear 0.5-72×88mm scope.” Why is that? Because, as you alluded, at a certain quality level (short of the Super ED) the differences are so small as to be indistinguishable—? Or do the optical differences matter for only certain things such as dealing with shimmer? Thoughts?

I reprinted the above question from a great thread currently running, which I believed I had derailed enough.

sigfreund I will try to answer your questionsone by one. If I miss something, let me kow.

1- Super-ED as a minimum to help deal with mirage? Well, I first detected that something was different with respect to mirage attenuation with my first March scope which has ED lenses compared to the NF NXS of the same power and diameter that I had before. At Deon, they tested an ED scope vs a Super ED scope and they found a marked difference in mirage attenuation between the two. They did not compare a traditional glassed scope and an ED scope simply because they do not make traditional glassed scopes. If they had, I believe they would have noticed a difference. Let me just say that when the mirage roamed, I had to lower the magnification on the NF, period, end of story. So the answer to your question is that I believe there is a benefit for ED glass over traditional glass in mirage attenuation. But it REALLY shows up with Super ED and CaF2 glass.

2- Minimum range to target for this effect to show? I do all my shooting these days at either 100 yards (rarely, just for load development,) or 1000-yards, with the occasional foray to 600 yards. I have never seen a difference, I'm always at 50X. I would think that if mirage is degrading your IQ at any distance, Ed, Super ED or CaF2 will make a difference.

I know DLO, ILya. I have met him several times and we correspond on and off. He knows far more about optics than I do, but he does recognize that I speak from experience at long range competition and dealing with that environment. I have deep respect for his extensive knowledge of optics and various riflescopes. When he talks, I listen. Well perhaps I may stray when it comes to aiming at the LR-1FC target at 1000 yards. That's when Super ED glass rules and lets you aim surgically.

With respect to your last paragraph, I will state that I have virtually no experience with that. I have shot a couple of PRS matches with a March-FX 4.5-28X52 HM with Super ED glass and had no problem finding the targets. Hitting them in the time alloted and from the position required was, problematic but that has nothing to do with optics.

But in the end, there are several aspects to optics that need to be kept in mind. You have resolution, aberration, magnification, color fidelity, reliability, controls, etc. Some of the traditional glass provides excellent resolution and the coatings will ensure color fidelity. Of course, mirage will degrade the IQ, so you deal with that by lowering the magnification and rely on the resolution of the glass. But if you have high quality Super ED glass, it will outperform the traditional glass in mirage conditions.

Thanks much for taking the time to respond to all that.

To be clear I certainly understand why being able to see the target clearly enough to achieve a precise point of aim is critical for precision shooting. Much of my interest in long range shooting is satisfied by watching videos of “accuracy” tests and it makes me a little crazy to start watching one and see the shooter use targets that are something like a big black bull’s-eye circle without even an X in the middle to help eliminate aiming errors. In fact, it’s so common that I must believe that most people simply don’t understand the concept—or perhaps they don’t really care about what they’re doing except to get a video posted, and assume that most viewers don’t know any better.

Your comments about how different types of glass help see through mirage effects have really piqued my interest. I have scopes with a range of types, an older Leupold Mark 6, a Mark 5 that has “HD” in its name, a couple of expensive models that don’t advertise anything about the glass used, and a March 4.5-28. As I mentioned before, I should see if I’m able to detect the differences in a side by side test before summer is gone in a few weeks.

Again, I really do appreciate learning from someone who knows what he’s talking about, especially if it corrects something I thought I knew. So thanks again for taking the time to try to explain some things.

sigfreund I would urge you to reread the article the link to which I posted a few entries back. In it, Deon explains that the shimmer protection effect appears to be a combination of things. The main "ingredient" is Super ED glass, installed in a longish scope.

My understanding is that you must have ED glass, and the closer to CaF2 it is in CA-control properties the better. The shimmer protection effect of the Super ED glass will be enhanced by a longer design.

The March-FX 4.5-28X52 HM is a short, compact design. I have not had any problems with mirage the few times I have used it at longer distances, at 28X. The vast bulk of my experience with mirage-induced IQ degradation has been looking at the LR-1FC target face at 1000 yards at 40X or 50X. I have looked at it with my 4.5-28X52 but could not see any corruption of the target face at 28X. The folks who dial down because their high-mag riflescopes cannot handle the mirage at higher mag, will go to low 30s or mid-to-high 20s, so that's not a great test. On the other hand, the 4.5-28X52 does show nicely the waves of mirage and that's important info to the shooter.

Thanks for mentioning the article. I reread it and (I believe) I understood the discussion of the chromatic aberration correction. I’ll have to look at it again, but why correcting for CA would affect how shimmer affects the IQ is a little vague to me at present. But if the effect was not anticipated, then perhaps the reason why it works to do that isn’t all that well understood even to the company experts.

I snatched this definition from the internet, but it expresses the problem simply. Chromatic aberration is an effect that occurs when a lens is not able to properly refract all the wavelengths of colour in the same point.

In terms of scope optics, super ED glass transmits more of the full color spectrum which allows the user to see through the mirage without reducing magnification. At least that's how I interpret the application. I can see how this was an unintended feature rather than a design option as rifle optics manufacturers don't look at true color reproduction characteristics the same way as photo lens manufacturers do.

quote:

Originally posted by SgtGold:
I snatched this definition from the internet, but it expresses the problem simply. Chromatic aberration is an effect that occurs when a lens is not able to properly refract all the wavelengths of colour in the same point.

In terms of scope optics, super ED glass transmits more of the full color spectrum which allows the user to see through the mirage without reducing magnification. At least that's how I interpret the application. I can see how this was an unintended feature rather than a design option as rifle optics manufacturers don't look at true color reproduction characteristics the same way as photo lens manufacturers do.

I like your thinking, but I have to correct a few things. As I mentioned before, pure fluorite crystal glass is inherently super ultra low dispersion. This type of glass does not exhibit CA, color fringing, color spreading, whatever you call it. It essentially focuses all refracted wavelengths properly. That type of glass does not lend itself well to riflescope applications because it's expensive, fragile and susceptible to changes in conditions (hot and cold.) It is found in very high end camera lenses and in a very few spotting scopes, like my Kowa 883. ED glass was developed 50+ years ago to approximate the CA control of CaF2 glass; it's a step in the right direction but ED glass is not CaF2 glass. Super ED glass was developed more recently and incorporates a lot of CaF2 and it gets closer to pure fluorite glass, very close.

So, compared to ED glass, and especially traditional optical glass, Super ED is able to refract all wavelengths of the visible spectrum very closely together, almost as well as CaF2 glass.

Now, let's talk coatings. Uncoated glass will reflect about 5% of the incoming light per air-to-glass boundary. Coatings were developed to be applied to the glass so as to reduce, almost completely eliminate the reflection of the light. However, coatings work for specific wavelengths. So if you only have one coating and let's say it's for blue, you will see that your image will appear bluish because blue does not lose 5% at every air-to-glass boundary. When you add up the reflection of other colors at each lens, you can see where the blue will really show. So manufacturers apply multiple coatings to deal with various colors in an attempt to get the best color reproduction. If you think glass provenance is a closely guarded secret, let me tell you about coatings. Or not. That is even more secretive.

To recap: Super ED glass to control color wavelength dispersion and coatings to control color wavelength transmittance.

My personal hypothesis is that non Super-ED glass allows all wavelengths to disperse, and the effect of mirage scrambles the image even more and that's why non Super-ED glass optics need to be dialed down in heavy mirage. Super-ED and CaF2 glass are not immune to mirage distortion, but they control the scrambling better. So to speak.

I thought I said that, just with a lot less supporting info. About 30 years ago I sold high end photography equipment. The concepts of glass quality, coatings, and light transmission are well known to me.

quote:

Originally posted by NikonUser:

quote:

Originally posted by SgtGold:
I snatched this definition from the internet, but it expresses the problem simply. Chromatic aberration is an effect that occurs when a lens is not able to properly refract all the wavelengths of colour in the same point.

In terms of scope optics, super ED glass transmits more of the full color spectrum which allows the user to see through the mirage without reducing magnification. At least that's how I interpret the application. I can see how this was an unintended feature rather than a design option as rifle optics manufacturers don't look at true color reproduction characteristics the same way as photo lens manufacturers do.

I like your thinking, but I have to correct a few things. As I mentioned before, pure fluorite crystal glass is inherently super ultra low dispersion. This type of glass does not exhibit CA, color fringing, color spreading, whatever you call it. It essentially focuses all refracted wavelengths properly. That type of glass does not lend itself well to riflescope applications because it's expensive, fragile and susceptible to changes in conditions (hot and cold.) It is found in very high end camera lenses and in a very few spotting scopes, like my Kowa 883. ED glass was developed 50+ years ago to approximate the CA control of CaF2 glass; it's a step in the right direction but ED glass is not CaF2 glass. Super ED glass was developed more recently and incorporates a lot of CaF2 and it gets closer to pure fluorite glass, very close.

So, compared to ED glass, and especially traditional optical glass, Super ED is able to refract all wavelengths of the visible spectrum very closely together, almost as well as CaF2 glass.

Now, let's talk coatings. Uncoated glass will reflect about 5% of the incoming light per air-to-glass boundary. Coatings were developed to be applied to the glass so as to reduce, almost completely eliminate the reflection of the light. However, coatings work for specific wavelengths. So if you only have one coating and let's say it's for blue, you will see that your image will appear bluish because blue does not lose 5% at every air-to-glass boundary. When you add up the reflection of other colors at each lens, you can see where the blue will really show. So manufacturers apply multiple coatings to deal with various colors in an attempt to get the best color reproduction. If you think glass provenance is a closely guarded secret, let me tell you about coatings. Or not. That is even more secretive.

To recap: Super ED glass to control color wavelength dispersion and coatings to control color wavelength transmittance.

My personal hypothesis is that non Super-ED glass allows all wavelengths to disperse, and the effect of mirage scrambles the image even more and that's why non Super-ED glass optics need to be dialed down in heavy mirage. Super-ED and CaF2 glass are not immune to mirage distortion, but they control the scrambling better. So to speak.

Thanks for the additional discussion.

I seem to remember seeing photographic lenses being advertised long, long ago that had fluorite elements. They were insanely expensive (for someone in my income bracket at least), and now that I think of it they were evidently on the market for only a short time. Other than expense, and possibly fragility, I wonder now why they faded away (assuming they did?).

quote:

Originally posted by sigfreund:
Thanks for the additional discussion.

I seem to remember seeing photographic lenses being advertised long, long ago that had fluorite elements. They were insanely expensive (for someone in my income bracket at least), and now that I think of it they were evidently on the market for only a short time. Other than expense, and possibly fragility, I wonder now why they faded away (assuming they did?).

I just checked at the Nikon site and they show a few lenses with pure fluorite AND ED glass elements. The 800mm lens is $16,000. The 600mm is much more affordable at $13,000.

All right, then. Still out of my price range—at least for what I’d do with one.

quote:

Originally posted by SgtGold:
I thought I said that, just with a lot less supporting info. About 30 years ago I sold high end photography equipment. The concepts of glass quality, coatings, and light transmission are well known to me.


I snatched this definition from the internet, but it expresses the problem simply. Chromatic aberration is an effect that occurs when a lens is not able to properly refract all the wavelengths of colour in the same point.

In terms of scope optics, super ED glass transmits more of the full color spectrum which allows the user to see through the mirage without reducing magnification. At least that's how I interpret the application. I can see how this was an unintended feature rather than a design option as rifle optics manufacturers don't look at true color reproduction characteristics the same way as photo lens manufacturers do.

SgtGold
Perhaps I misunderstood your comment and that's on me. The way I read it what that you were saying Super ED was transmitting more of the full color spectrum, and I took that to mean "in comparison to traditional glass." And that's what I was discussing.

I was not aware that you sold high end photography equipment 30 years ago and thus knew about coatings, transmission and so on.

Since this is a stickied thread, I write my "technical" responses for everyone. So while you know about all that stiff, others who might read this thread may not and I try to explain it together. It's not a knock on you or your knowledge by any means.

This message has been edited. Last edited by: NikonUser,

Gotcha. I appreciate the depth of your knowledge on the subject.

quote:

Originally posted by NikonUser:

quote:

Originally posted by SgtGold:
I thought I said that, just with a lot less supporting info. About 30 years ago I sold high end photography equipment. The concepts of glass quality, coatings, and light transmission are well known to me.


I snatched this definition from the internet, but it expresses the problem simply. Chromatic aberration is an effect that occurs when a lens is not able to properly refract all the wavelengths of colour in the same point.

In terms of scope optics, super ED glass transmits more of the full color spectrum which allows the user to see through the mirage without reducing magnification. At least that's how I interpret the application. I can see how this was an unintended feature rather than a design option as rifle optics manufacturers don't look at true color reproduction characteristics the same way as photo lens manufacturers do.

SgtGold
Perhaps I misunderstood your comment and that's on me. The way I read it what that you were saying Super ED was transmitting more of the full color spectrum, and I took that to mean "in comparison to traditional glass." And that's what I was discussing.

I was not aware that you sold high end photography equipment 30 years ago and thus knew about coatings, transmission and so on.

Since this is a stickied thread, I write my "technical" responses for everyone. So while you know about all that stiff, others who might read this thread may not and I try to explain it together. It's not a knock on you or your knowledge by any means.