ED glass, CA and mirage

[Turbulent Turtle]

Since I can't shoot rifles at long range during the Wuhan virus pandemic, I can at least think about it.

So, while reflecting on the atmospheric vagaries than engender issue looking at a target on 1000 yards, I dedicate a few neurons to try to explain why I believe that ED glass helps tame the mirage in the picture presented to you eye. I think I have the inklings of the start of an hypothesis.

The reason ED (and Super-ED) glass is found in a riflescope (or other optics) is to minimize or eliminate the effect known as chromatic aberration, or CA. This is a phenomenon that is the result of bending the light in a lens and having the various wavelengths that make up the light focus a little bit apart from each other so that you get what is called "color fringing." This is where there is not a clear sharp demarcation between objects of different colors. This reduces the contrast and brings in other artifacts. The colors just don't pop. The rings on the target are furry, not sharp.

This CA thing manifests itself more the more you bend the light. The further from the exact center of the lens the object is, the more the light is bent. So as you look at the overall image, while the center may look really good, the image slowly degrades the further from the center it is; put another way the CA grows as you look at the image away from the center.

Now, when you are aiming at the target, the reticle will be placed on the target (I would hope,) but the reticle is at the center of the erector tube (FFP or SFP), and the erector tube may not be in the middle of the scope looking directly at the center of that big objective lens. In fact, if you have a lot of elevation on the scope. chances are your erector tube is approaching the top of the main tube and you are in effect looking through the bottom of that big objective lens, not the nice middle part.

When I look at the target line, I see the mirage running either just below the target line, or just above it and I definitely see it on the target line itself. Mechanically, I am looking at the portion of the image that is fed by the bottom of the objective lens; right where the CA would be more prominent than at the center of the lens. CA brings in color fringing and thus reduces contrast and blends the colors a little bit and the shimmer of the mirage enhances that effect to our eye.

By having ED glass in the scope, the color fringing is reduced or near eliminated and the shimmer of the mirage is on its own messing with the image; the multiplying effect of CA is reduced or eliminated and that's why I detect an improvement in IQ looking through mirage at the target 1000 yards away.

One of the things I have done is actually increase the cant of my Super-ED glassed riflescope to 30MOA to reduce the distance from the middle of the lens when shooting at 1000 yards. Super-ED glass AND closer to the middle; I can't wait for the next match and there will be mirage.

 

[Justin Credible]

Now, when you are aiming at the target, the reticle will be placed on the target (I would hope,) but the reticle is at the center of the erector tube (FFP or SFP), and the erector tube may not be in the middle of the scope looking directly at the center of that big objective lens. In fact, if you have a lot of elevation on the scope. chances are your erector tube is approaching the top of the main tube and you are in effect looking through the bottom of that big objective lens, not the nice middle part.

When I look at the target line, I see the mirage running either just below the target line, or just above it and I definitely see it on the target line itself. Mechanically, I am looking at the portion of the image that is fed by the bottom of the objective lens; right where the CA would be more prominent than at the center of the lens. CA brings in color fringing and thus reduces contrast and blends the colors a little bit and the shimmer of the mirage enhances that effect to our eye.

Every part of an image is formed by the entirety of the objective lens. Even if the lens is pointed directly at something the edge of the lens is still helping to form the image, and something at the edge of the field of view is still using the center and the entire edge to form the image. So you are never looking at the part of the image formed by the bottom, center, or any other single area of the lens.

There is more than one type of chromatic aberration and correcting one doesn't necessarily correct the others, and in fact certain ones are generally over/under-corrected for overall balance. Like some other aberrations, chromatic is usually more noticeable as you move further from center of the image. By the time you get out there you have your various chromatics, coma, and astigmatism all adding up to trouble.

Your overall premise may still be be correct, that the better color correction that you tend to get with ED glass makes mirage less visible, and adjusting the scope mount to view the center of the image is better in this regard. I never really put much thought into it. Another consideration is that companies that invest the money into lenses made from ED glass are also more likely to invest in better designs that limit other aberrations that tend to increase further off-axis. So that may be playing a role as well. It will be interesting to hear your results.

Justin

 

[Turbulent Turtle]

Thank you for the great reply, Justin. I always try to reduce complicated concepts into something that I can visualize and "understand". I find that a way for me to learn is to write about my thought process, try to explain it in terms that I understand and put it out there for others to confirm, infirm and correct and then I learn more and that helps me and, it is hoped, others along the way.

That's what I do in real life, so this is a professional deformation.

Yes, I understand about the image being formed by the entire objective lens, but as a long time avid photographer (50+ years) I know that imperfections on a lens will show up on the image. You can pick out artifacts on the image caused by a piece of snot on the lens. You might recall that in the late 60s-early 70s, the photographic world was getting into lens coating. (Well, not just the photographic world, but that was whence I was getting my exposure to lens coating.) Some of the coatings were better than others, or put another way, some were worse than others. I remember an off-brand lens that I bought, in which some of the coating on the objective lens seemed to "wash away" in one area of the lens. This was over 40 years ago, so don't ask for details. In those days, the university had darkrooms available for students; all you had to supply was the film (yes, film) and photo paper. This is where I could see the demarcation on the image where the "coating" was incorrect; that showed up nicely on the enlarger.

So while one may never really look through a certain part of the objective lens, specific parts of the objective lens will be showing up on your prints.

Thank you for pointing out there are various types of chromatic aberration. My understanding is that ED glass (and Fluorite crystal glass) absorb or somehow mitigate the secondary spectrum from the CA, so I am definitely not clear (pun intended) on how that is different for the various types of CA.

Also, my understanding of a riflescope design is that the image you get to your eye comes from whatever the erector tube "sees". So, let's say the ID of the main tube is 26mm. The erector tube may be something like 20mm in OD to provide 3mm up and down of travel or adjustment. (I'm just guessing here about the erector tube and the travel adjustment, I would have to do some math to figure out the exact dimensions, so bear with me.) The ID of the erector tube would probably be something like 18mm.

I wear glasses, been wearing glasses for 55 years or so. I can tell you that if I have a smudge on a lens, it show up especially if I look through the glasses where that smudge is. I believe that the erector tube facing the less perfect area of then objective lens may be equivalent.

One reason to believe that is that some optics company make a big deal about the optics being centered on the sweet spot. For instance, March makes a riflescope model called the Genesis. One of its features is the genormous adjustment range of the scope due to the fact the adjustment is external, not internal. Because of that, the image coming through the riflescope is always through the middle of the lens, the best image possible. And they tout that feature; you always get the best image regardless of the adjustment in the riflescope.

I have only shot with my latest setup in two matches before the Chinese virus caused everything to be shut down. I did write up how incredible the image of the 1000 yard target was on the latest foray with my setup. I had never seen the target so well in 15 years shooting at Bayou Rifles 1000 yard line as that one day. It was an exceptional day to be sure, but the riflescope setup took full advantage of it. I am really looking forward to the day we can compete again and when the mirage will come out to play with the IQ from our riflescopes. I will certainly report back.

 

[Justin Credible]

I can imagine when you magnify an image, as would be done with an enlarger for film, there might be some visible artifact from large enough snots on the lens. On most optical surfaces for imaging you can have fairly large scratches and digs on the surface an not affect performance, which is why we often call them beauty specs, but snot size could be an issue! Generally the closer they are to an image plane the more harmful they are, but there are exceptions to that.

I'm a lens maker and not a designer, though I do know some design stuff, so I don't really have the knowledge to go any deeper into chromatic aberrations. I think the secondary spectrum is better with ED glasses simply because they require a less dramatic correction with another lens, and so they overshoot the wavelengths in between the corrected colors less. I do not know if that affect is the same with all of the various chromatic issues.

Your understanding of the erector tube diameter is the same as mine, and I don't know the exact dimensions either! I guess it depends on how much elevation/windage travel they decide to give you along with metal thickness.

Speaking of lenses, today while getting ready to run some aspheric crowns for an achromat, which I believe is part of an eyepiece, it hit me that in the eyepiece only part of the lens is used to transmit the image for any point in your eye. That is the lower edge of the lens creates the lower edge of the image, etc. So having a properly manufactured lens in the eyepiece can be a big deal here. In the objective lens, and most other lenses in the system, you can have a fairly large shape error of just about any shape and not really affect the image quality. In an eyepiece you can take the same level of error but if it concentrated in one area, say the edge "falls off", then this part of the image will suffer. There may also be higher aberrations at the edge due to design trade-offs which affect edge quality as the center is the important part. Another thing to think about with your testing!

Justin

 

[Turbulent Turtle]

I always considered the eyepiece in a riflescope as some sort of "enlarger," believing that it magnifies the merged image of the reticle in the plane of the target. There are a few scopes that have the zoom in the eyepiece just like in a spotting scope, at least the ones with a zoom eyepiece.

So, following that thinking, I would think the eyepiece will simply amplify any "defect" coming from the objective lens.

Texas has taken the first steps to reopening and the ranges are now open but there are no competitions. I don't know that I can wait much longer and may just pop over some morning.

 

[jaytee]

Great info guys! I'm just a curious onlooker as far as competition shooting goes but this info is great for anyone who likes to burn some powder at the range or in the field! So my guess here is that ED glass is only found on the high end scopes. Does anyone make an entry or mid-level scope for casual target shooters or hunters that use ED glass, say something in the 4-600 dollar range?

 

[Raythemanroe]

I have a 30 MOA on my last 1k BR rig with ED glass. I feel having it more optically center is a plus. I also know that it's not good for a scope to be run at/near the limit of adjustment all the time. At least that is how a few optics companies have explained it.

Ray

 

[Turbulent Turtle]

Great info guys! I'm just a curious onlooker as far as competition shooting goes but this info is great for anyone who likes to burn some powder at the range or in the field! So my guess here is that ED glass is only found on the high end scopes. Does anyone make an entry or mid-level scope for casual target shooters or hunters that use ED glass, say something in the 4-600 dollar range?

The only decent sub-kilobuck scope with ED glass that I know of was now-discontinued Nikon Monarch 5. I think it was a 5, but I can't recall. On other sites, I suggested that scope to a couple of people and they loved it. I do not have one myself, but in retrospect, maybe I should have picked one up. It was around $500, IIRC.

In my defense, all my shooting these days (or until about a month ago, prior to the Wuhan-induced lockdown) has been LR competition focused. I have plenty of other scopes for my non-match rifles but that one would have been nice to snap up.

 

[Turbulent Turtle]

This weekend, we had our first practice match since the Wuhan flu shut down all comps back in March. I brought two scopes to the range; my March-X 5-50X56 and my March-X 10-60X56 HM. I had the 5-50X56 installed on a tripod with the March accessory kit that transforms a scope into a spotting scope. The 10-60X56 HM is on my F-TR rifle. I shot horribly, marksmanship is a perishable skill and chastised myself for not spending time dry firing during the lockdown. However, I had a chance to play with the magnification and the weather was very nice. It was windy with fishtailing winds in our face and from the right and such. The mirage was not apparent in the early part of the match, but did start manifesting itself an hour or so into the match. I could see it quite clearly in my Kowa, non-ED glass spotting scope, but it was very hard to discern in my HM scope. I cranked up the magnification from my usual 40X to 50X and even 60X for a little while. I backed it down to 50X as I found that it was a bit dark at 60X and jumping a little bit more than I liked when touching the rifle. 50X is now my new normal.

At the height of the heat and mirage, the wind was still strong enough to keep the mirage low and moving so there were no boils, but I did notice that the target face was not as crisp as it had been earlier in the day, this was at 50X. I was thinking of installing an modifier disk to see if that would help because it was really bright out yesterday and dropping 50% of the light (1 f-stop) might help, I discovered I had left the MD unit in my vehicle and I was too lazy (heat was pretty bad) to go get it.

We use e-targets and we had been working on them over the last week. We made a bunch of new frames and targets and we wrote the dimension on the target with a Sharpy. This is a block of text at the bottom right of the target. At 50X with my HM, I could actually see the block of text and I could tell what it was. I could not read it. This was impressive to me. The letters and numbers are about 1 inch high.

Back on the tripod with the 5-50X56, I tried to see if I could discern the block of data, and while I could see "something", I could not tell what it was. So advantage High Master (Super ED) Lens system over ED lens.

 

[F Class John]

Great information and fascinating to consider what you’ve bantered back and forth about.

The part that’s frustrating as a consumer is that you may never know how many and which lenses are ED. All that’s required for any scope manufacturer to label their box ED is to have a single piece of ED glass in the scope. So when you see ED on a box it could mean anywhere from one to all are ED and which one(s) they choose to make ED could all be different. Sadly some companies use it as marketing and simply pick the easiest and cheapest lense inside to make ED while others chose the lense(s) that will be most impacted by changing to ED. Unless you talk to the manufacturer and are fortunate enough for them to be honest about it you’ll never know...

 

[Turbulent Turtle]

An excellent point FCJ, and thank you for the kind thoughts.

I asked similar questions of March a while back. Let me preface this by restating (ad nauseam, I'm sure) that I am a long time avid photographer. ED lens elements found their way into camera lenses starting in the late 60s when Nikon (Nikkor) developed ED glass as a fair substitute for fluorite crystal glass elements. The latter are expensive, difficult to make and fragile. ED glass, while not as good for CA as pure fluorite crystal glass, went a long way to correct CA. Nikon (Nikkor) documents their lenses and you can find out easily enough how many glass elements and groups make up a lens and which ones are ED, Super-ED and even fluorite crystal. The location and number of these elements vary by lenses and I haven't researched it enough to say I understand their locations and so on. (In other words, I haven't a clue, I just treat it as information.)

So back to the question I posed to March. I asked them how many and which lenses are ED glass in their design and the same for Super-ED glass. The answer I got was that in the riflescopes with ED glass, there is a single ED glass element and it's the front objective lens. In riflescopes with Super-ED lenses, the High Master Lens series, there are two Super-ED glass elements, the doublet at the front; the objective lens and the one right behind it. That helps explain the price difference and why the HM series of riflescopes have such spectacular IQ.

 

[Justin Credible]

Great information and fascinating to consider what you’ve bantered back and forth about.

The part that’s frustrating as a consumer is that you may never know how many and which lenses are ED. All that’s required for any scope manufacturer to label their box ED is to have a single piece of ED glass in the scope. So when you see ED on a box it could mean anywhere from one to all are ED and which one(s) they choose to make ED could all be different. Sadly some companies use it as marketing and simply pick the easiest and cheapest lense inside to make ED while others chose the lense(s) that will be most impacted by changing to ED. Unless you talk to the manufacturer and are fortunate enough for them to be honest about it you’ll never know...

As TT said, in rifle scopes with ED glass there are generally just one or two elements (out of ~dozen) that are made from it (as far as I am aware). And that is actually a good thing. A scope with all ED glass would have some pretty bad chromatic aberration (color fringing). Even CaF2 has some dispersion, which causes this color, and would not look good. You'd think it was a cheap scope. The best solution is an achromat, which rifle scope objectives (and eye pieces) generally are. This uses a lower dispersion positive element combined with (usually with matching curves cemented together) a high dispersion negative element. By choosing the proper combinations of the two glasses (there are several 'classic' combos, but the requirements of each system are unique) this achromatic doublet can have less chromatic aberration than even the most super-duper low dispersion glass or crystal. What the ED glass does is allow for some more interesting choices of glasses that allow for a better color correction than traditional glasses, so a properly designed achromat with an ED glass can have better performance. Note that the designer may not try to fix all of the chromatic with the one doublet (or triplet), it may be over or under corrected and to counteract what a later lens in the system is doing. This goes for all aberrations.

ED glass generally makes the scopes more expensive as the glass itself is more expensive and it is generally more difficult to grind and polish so labor costs go up. That being said, probably the most expensive glasses I have ground and polished actually have very high dispersion but other properties of the glass make it very useful for the system it goes in. Optical design is all about compromise throughout the system, which is why there are so many different glasses.

Justin

 

[Turbulent Turtle]

Even camera lenses only have a few of the glass elements made of ED glass or Fluorite crystals. Just for grins, I looked at the longest lens available from Nikon, the AF-S Nikkor 800mm f/5.6 FL ED VR. It has 20 elements in 13 groups, of which 2 elements are fluorite crystals, the objective lens and its associated doublet partner, and 2 ED lens elements elsewhere. It's only $16,000 and weights 10 pounds. Another on is the AF-S Nikkor 180-400mm F/4E TC 1.4 FL ED VR. It has 27 elements in 19 groups, of which there are 8 ED elements and one fluorite element. It's much cheaper at $12,400 and only weighs 7.5 pounds.

 

[Bob L.]

Ouch. I’m glad I never got into photography.

 

[Turbulent Turtle]

Ouch. I’m glad I never got into photography.

Yeah, those lenses are for hard core professional photographers. They make very few of those lenses to begin with. Also, lots of pros will rent such a lens for an assignment and send it back to the lens rental place at the end of the assignment.

Other examples would be where a studio or some such would own one of these lenses and would issue it to a photographer for an assignment.

 

[JFrank]

Fascinating!
Thanks Gents for a really
interesting read.
J

 

[KY_Windage]

The mirage is heat waves that bend the rays of light as they travel from the target to the scope, so I don't think it matters what your lenses are made out of. But, hey, I'm just a shooter so I could be wrong.

The waves were not bad at 600 yds when we started yesterday at 10:00, but in the middle of the afternoon they were. I just wait for a lull if I can, average 'em out if I can't. The problem is I have to wait for the lulls in the breeze, too, but the mirage gets worse when the breeze lays.

Rifle was a factory 700 Varmint w/ a 15-55 Comp on it. I cranked the mag ring until the picture seemed to be ideal, and that turned out to be 47x, close to the 50x you like. All shots except the one low one went into about a 7" circle, so I was happy. I knew I had one in the box that was short on powder but I had no way to find it other than by shooting.

One of the holes on the left side on the red bull was already there, so it looks like I fired about 18 rounds. The back nine were much better than the front nine, proving I needed the practice.

 

[Turbulent Turtle]

We had a 1000 yard match this weekend, and I took the time to explore further the capabilities of my March-X 10-60X56 HM. During the first two relays, the sky was cloudy so there was no mirage. I boosted the magnification to 50X and show the first two relays with that setting. I really enjoyed doing that as in the field of view, all I saw was my target and enough left over to see the target number below the target line. I could see some of the target on either side of mine, but since we use electronic targets, watching the other targets in the scope is useless. The picture was very clear and I could place the dot of my MTR-5 reticle exactly where I wanted it.

The sun burned through the clouds during the morning so that by the time the third match started, the mirage was coming out. I was third relay, wiping up the rear, and the mirage was out by then. I stayed at 50X, and while I could clearly see the mirage below the target line, the heat shimmer was not degrading the image of the target very much at all. I really think the twin Super-ED lenses in the riflescope, coupled with staying near the center of the lens with the use of a 30 MOA canted rail is helping to deal with the mirage quite nicely. So far. Let's see if I stay at 50X in August.

 

[Ranger188]

You should have looked thru a couple of other scopes at that time to compare how your looked to
the others with less quality glass thru the mirage.
I'm sure someone had a GE or SIII

 

[JEFFPPC]

You should have looked thru a couple of other scopes at that time to compare how your looked to
the others with less quality glass thru the mirage.
I'm sure someone had a GE or SIII

You have missed the narrative.