ED glass, CA and mirage
- Thread starter Turbulent Turtle
- Start date
With the 30 minute rail are you above or below optical center at 1 thousand yards?
Turbulent Turtle
F-TR competitor
I'm just a tad (2 MOA) above center according to adjustment range. In other words, I'm about as close to center as I can hope to be for my 1k zero at sea level with my heavy bullets. It's almost as if I had planned it that way.
The next barrel you put on that will all change...... jim
Turbulent Turtle
F-TR competitor
I'm sure it will change some but it's not going to be difficult to adjust for it, after all, I'm right in the middle of the range; right where I want to be.
ronsatspokane
Gold $$ Contributor
Hate to tell ya this gentlemen but you are just scratching the surface with respect to the science of optics. The issues are many. The types of aberrations are significantly greater than just chromatic aberration. Let consider a few then get back to chromatic.
Non-chromatic aberrations such as defocus, spherical, coma, astigmatism, barrel, pincushion, mustache, field curvature and others also need to be taken into account and corrected (as best they can be). These are addressed in the design and manufacturing phases. There are tools for the design phase and automated lens grinding machines but the best lenses are still ground by hand and tested throughout the process. The impact of these on the image can be significant and in high end camera lenses and telescopes they are often well controlled. When people talk about edge to edge sharpness, these non-chromatic aberrations are often the root cause if it is not good.
Percent of light transmission through the glass elements, the higher the better. This is partially addressed via coatings with the number of coatings and surfaces coated being relevant. There is a significant difference between multi-coated and fully multi-coated. Fully multi-coated implying that all surfaces are coated. The chemical composition of the coatings used and the number of coatings on which surfaces are often a trade secret, especially in high end optics. Coatings cut down on the internal light reflection and that in turn means greater contrast in the image. When coupled with edge blackening and non-reflective coatings on the internal tube surface itself, light transmission can reach 99% in high end optics. The impact of coatings on the image are generally in the area of washed out images due to internal reflections and lack of contrast.
Resolving power. The ability to separate small details in an optical image is referred to as resolving power. It is usually measured as a theoretical limit given that all of the aberrations are perfectly corrected. They never are but being able to see the grains of paper at 50x in a scope with a 80mm objective lens versus a scope with a 30mm objective at the same power tells the story. Assuming the same optical quality, the larger lens will resolve better than the smaller one. In a telescope it may make the difference between resolving the Cassini division in the rings of Saturn or not. The Cassini division is about .75 arch seconds wide. A scope capable of resolving it would also need the quality coatings that provide the necessary contrast to see it in addition to all other aberrations being well controlled and also in addition to an objective that provides the theoretical resolving power necessary.
Image correction (or erection if you will). When talking about rifle scopes and "some" refractor telescopes there are erector lenses in the lens assembly and the issue is, the more glass the more potential for aberrations. In spotting scopes, binoculars and camera view finders you also have prisms for erection of the image. These devices can be very good or they can introduce their own aberrations into the image. High quality BAK4 prisms are generally considered the best but quality erector lenses within the OTA (optical tube assembly) are better.
The atmosphere itself. The atmosphere is a lens. When people talk about mirage or heatwaves, what they are seeing is the impact of light traveling through that atmospheric lens. You will never (with the depth of most pocket books) be able to fix that. The best you can hope for is to see it absent aberrations within the lens assembly further distorting it. When I hear people look through a cheap scope and say that there is less mirage I shake my head. You buy expensive glass so that you can see it unadulterated rather than obscured as happens in cheap optical assemblies. I have a very nice ED astronomical scope that I've used as a spotting scope in the past that shows the most beautiful mirage. Incredible detail that makes it look like a river flowing through the ether. When sitting side by side with a 3000 dollar Kowa fluorite APO (the best of the best in spotting scopes) it is very close to what you will see in that. There is an exception to the above. Active optics can correct for the atmospheric lens. Ground based telescopes can "approach" the image quality of space based telescopes but at a price point that only governments can afford.
OK, back to chromatic aberration. Chromatic aberration is caused by the failure to focus all wavelengths of light to the same point. All glass and all other elements used in OTA's have this problem. Some glass/elements have less of a problem. Crown and basic flint glass have the biggest problem. This is what cheap optics are made of. ED glass has less of a problem. There are many types of ED glass and aside from those who supply raw glass (like Schott out of Germany), the chemical composition of that type of glass is usually a closely held secret. Then there are crystals like Fluorite that have the least problem. Fluorite is the commonly used high end optical element and those who use it generally produce very high quality products. High end camera lenses like Canon L series, high end spotting scopes like Kowa and high end telescopes like Takihashi all use it. It minimizes the number of lenses and/or the degree of correction required to refocus the light to a common point. All types of glass can be corrected (to a degree) with additional lenses but the problem becomes more difficult in materials that disperse the wavelengths the most and as with everything else, the more glass you use to correct the problems of the other glass, the more potential aberrations of all types you introduce.
The above just scratches the surface of optical science is serves as a takeoff point for further research. The subject gets deep fast. I am not an optical scientist or even an optical engineer, I'm only a glass snob who has looked into the subject quite a bit over the years. I've run Takahashi Fluorite APO refractors for amateur astronomy in years past but sold those off and currently only use a portable ED APO refractor when the mood strikes. It's a very good scope but does not stand up to Takihashi. I do currently run Canon L series lenses for photography. They are very good. Much better than the standard line of lenses in the images they have produced. Most of my other optics are ED where it is important (like in F Class) but I do own cheaper scopes as well. I'm not rich so I can't have the best of everything and even the best optical equipment has its aberrations. It all comes down to budget and how many of which kind of aberrations you are willing to tolerate. When my pockets are empty, I can tolerate more than when my pockets are full and with optics, you do get what you pay for. There are no shortcuts. There are only aberrations that you can live with and those you can't. The more trained the eye, the fewer you can live with and the more empty your pockets become.
Non-chromatic aberrations such as defocus, spherical, coma, astigmatism, barrel, pincushion, mustache, field curvature and others also need to be taken into account and corrected (as best they can be). These are addressed in the design and manufacturing phases. There are tools for the design phase and automated lens grinding machines but the best lenses are still ground by hand and tested throughout the process. The impact of these on the image can be significant and in high end camera lenses and telescopes they are often well controlled. When people talk about edge to edge sharpness, these non-chromatic aberrations are often the root cause if it is not good.
Percent of light transmission through the glass elements, the higher the better. This is partially addressed via coatings with the number of coatings and surfaces coated being relevant. There is a significant difference between multi-coated and fully multi-coated. Fully multi-coated implying that all surfaces are coated. The chemical composition of the coatings used and the number of coatings on which surfaces are often a trade secret, especially in high end optics. Coatings cut down on the internal light reflection and that in turn means greater contrast in the image. When coupled with edge blackening and non-reflective coatings on the internal tube surface itself, light transmission can reach 99% in high end optics. The impact of coatings on the image are generally in the area of washed out images due to internal reflections and lack of contrast.
Resolving power. The ability to separate small details in an optical image is referred to as resolving power. It is usually measured as a theoretical limit given that all of the aberrations are perfectly corrected. They never are but being able to see the grains of paper at 50x in a scope with a 80mm objective lens versus a scope with a 30mm objective at the same power tells the story. Assuming the same optical quality, the larger lens will resolve better than the smaller one. In a telescope it may make the difference between resolving the Cassini division in the rings of Saturn or not. The Cassini division is about .75 arch seconds wide. A scope capable of resolving it would also need the quality coatings that provide the necessary contrast to see it in addition to all other aberrations being well controlled and also in addition to an objective that provides the theoretical resolving power necessary.
Image correction (or erection if you will). When talking about rifle scopes and "some" refractor telescopes there are erector lenses in the lens assembly and the issue is, the more glass the more potential for aberrations. In spotting scopes, binoculars and camera view finders you also have prisms for erection of the image. These devices can be very good or they can introduce their own aberrations into the image. High quality BAK4 prisms are generally considered the best but quality erector lenses within the OTA (optical tube assembly) are better.
The atmosphere itself. The atmosphere is a lens. When people talk about mirage or heatwaves, what they are seeing is the impact of light traveling through that atmospheric lens. You will never (with the depth of most pocket books) be able to fix that. The best you can hope for is to see it absent aberrations within the lens assembly further distorting it. When I hear people look through a cheap scope and say that there is less mirage I shake my head. You buy expensive glass so that you can see it unadulterated rather than obscured as happens in cheap optical assemblies. I have a very nice ED astronomical scope that I've used as a spotting scope in the past that shows the most beautiful mirage. Incredible detail that makes it look like a river flowing through the ether. When sitting side by side with a 3000 dollar Kowa fluorite APO (the best of the best in spotting scopes) it is very close to what you will see in that. There is an exception to the above. Active optics can correct for the atmospheric lens. Ground based telescopes can "approach" the image quality of space based telescopes but at a price point that only governments can afford.
OK, back to chromatic aberration. Chromatic aberration is caused by the failure to focus all wavelengths of light to the same point. All glass and all other elements used in OTA's have this problem. Some glass/elements have less of a problem. Crown and basic flint glass have the biggest problem. This is what cheap optics are made of. ED glass has less of a problem. There are many types of ED glass and aside from those who supply raw glass (like Schott out of Germany), the chemical composition of that type of glass is usually a closely held secret. Then there are crystals like Fluorite that have the least problem. Fluorite is the commonly used high end optical element and those who use it generally produce very high quality products. High end camera lenses like Canon L series, high end spotting scopes like Kowa and high end telescopes like Takihashi all use it. It minimizes the number of lenses and/or the degree of correction required to refocus the light to a common point. All types of glass can be corrected (to a degree) with additional lenses but the problem becomes more difficult in materials that disperse the wavelengths the most and as with everything else, the more glass you use to correct the problems of the other glass, the more potential aberrations of all types you introduce.
The above just scratches the surface of optical science is serves as a takeoff point for further research. The subject gets deep fast. I am not an optical scientist or even an optical engineer, I'm only a glass snob who has looked into the subject quite a bit over the years. I've run Takahashi Fluorite APO refractors for amateur astronomy in years past but sold those off and currently only use a portable ED APO refractor when the mood strikes. It's a very good scope but does not stand up to Takihashi. I do currently run Canon L series lenses for photography. They are very good. Much better than the standard line of lenses in the images they have produced. Most of my other optics are ED where it is important (like in F Class) but I do own cheaper scopes as well. I'm not rich so I can't have the best of everything and even the best optical equipment has its aberrations. It all comes down to budget and how many of which kind of aberrations you are willing to tolerate. When my pockets are empty, I can tolerate more than when my pockets are full and with optics, you do get what you pay for. There are no shortcuts. There are only aberrations that you can live with and those you can't. The more trained the eye, the fewer you can live with and the more empty your pockets become.
Fred Bohl
Gold $$ Contributor
One of my little three man optics testing group actually designed telescopic sights before he retired. He is predicting that the next approach to correcting aberrations will be to use an aspheric surface on one or more lenses. First in the erector assembly and then in the parallax adjust cell.
Turbulent Turtle
F-TR competitor
Aspherical lenses have already found their way in camera lenses. But even with such elements present, Nikon still includes one or more ED and/or Super ED and/or Fluorite crystal lens elements in some of their lenses. The fun ones are aspherical ED lens elements.
The other Nikon innovation that I was wondering if it would ever show up in a riflescope is Phase Fresnel lenses, again to combat chromatic aberration. I don't think that's going to happen any time soon. It would make for lighter, smaller riflescopes but the cost is prohibitive.
The other Nikon innovation that I was wondering if it would ever show up in a riflescope is Phase Fresnel lenses, again to combat chromatic aberration. I don't think that's going to happen any time soon. It would make for lighter, smaller riflescopes but the cost is prohibitive.
There are rifle scopes out there with aspherical lenses, though the last one I saw advertised as such was at such a low price they must be molded plastic aspheres, which will kill performance. There are more than one prototype stage rifle scopes with precision glass aspherical optics in them, I have ground and polished them myself! There may be some production scopes out there with them as well, but I do not know for certain. The big problem is cost. That single aspheric element will cost hundreds of dollars just by itself. Precision glass molding can reduced that cost when the volume is big enough (very high initial tooling costs) but has limitations on precision (but which has actually gotten quite good) and the limited selection of glasses that can be molded.
Justin
Aspherical lenses are not generally useful for correcting chromatic aberrations, so they will not negate the need for ED glasses (there are some very limited, very special case exceptions to that). I have made many aspherical achromats (one element is spherical on both sides, one has its outer surface as an asphere) for various optical devices, some of which use ED glass for one of the elements. I have even, many moons ago, polished some very large calcium fluoride (fluorite) aspheres. That is an interesting material to polish, to say the least. That's a few dollars worth of a lens for you!
Justin
Turbulent Turtle
F-TR competitor
Understood. I was just quoting from the Nikon website about the use of aspherical lenses. I was surprised to see they had aspherical lenses made with ED glass, which is why I specifically mentioned it. I'm always learning. Thanks for the note.
Fred Bohl
Gold $$ Contributor
In order to clarify and/or simplify my post #29 above, the purpose for aspheric lens surface is to correct for the spherical aberration caused by using spherical surfaces on lenses. See also:
Justin,
I was recently reading a tech note that said small aspheric surfaces could be made by new selective surface coating methods on a spherical surface at lower production cost than grind/polish methods.
Justin,
I was recently reading a tech note that said small aspheric surfaces could be made by new selective surface coating methods on a spherical surface at lower production cost than grind/polish methods.
Fred,
Thank you for sharing that image. You must have been reading my mind earlier as I had to give an asphere tutorial to a coworker today and drew the same thing on a piece of paper for him... but I don't draw so well! Aspheres can be used to correct other aberrations as well, but spherical is the primary one they go for. Most aren't as extreme as the image either, but it's good for demonstrating what one actually is. The one I drew earlier was even a little more extreme!
My professional life (~20 years) has all been with grinding, polishing, and measuring precision lenses (spheres, aspheres, and plano) so I only have secondary knowledge of molding, coating, etc. I am aware of aspheres made by taking a spherical surface and molding a plastic aspheric surface onto it, though I have no first hand experience with one. Maybe that is the coating you are referring to? Edmund Optics has such lens available in their catalog https://www.edmundoptics.com/f/aspherized-achromatic-lenses/13532/ I'm guessing they have fairly low precision by how they word the specifications, but have never measured one. Certainly the surface is less durable than glass. But for some applications they may work great at a reduced cost. Not every lens needs super-duper precision (just measure your eye glasses, very ugly).
Justin
Thank you for sharing that image. You must have been reading my mind earlier as I had to give an asphere tutorial to a coworker today and drew the same thing on a piece of paper for him... but I don't draw so well! Aspheres can be used to correct other aberrations as well, but spherical is the primary one they go for. Most aren't as extreme as the image either, but it's good for demonstrating what one actually is. The one I drew earlier was even a little more extreme!
My professional life (~20 years) has all been with grinding, polishing, and measuring precision lenses (spheres, aspheres, and plano) so I only have secondary knowledge of molding, coating, etc. I am aware of aspheres made by taking a spherical surface and molding a plastic aspheric surface onto it, though I have no first hand experience with one. Maybe that is the coating you are referring to? Edmund Optics has such lens available in their catalog https://www.edmundoptics.com/f/aspherized-achromatic-lenses/13532/ I'm guessing they have fairly low precision by how they word the specifications, but have never measured one. Certainly the surface is less durable than glass. But for some applications they may work great at a reduced cost. Not every lens needs super-duper precision (just measure your eye glasses, very ugly).
Justin
Turbulent Turtle
F-TR competitor
I don't think that spherical aberration is contributing to the distortion engendered by mirage when using a riflescope. I believe that CA is more to blame in that respect.
Nope, I can't explain.
Also, now I understand why Nikon makes aspherical lenses with ED glass.
Nope, I can't explain.
Also, now I understand why Nikon makes aspherical lenses with ED glass.
ronsatspokane
Gold $$ Contributor
Just out of curiosity what applications are you grinding lenses for? That's an art. One that you don't run across many people who are involved in it. I'm sure there are a lot out there but the only person I've heard of for whom it was an occupation was Baruch Spinoza. He also happens to be an interesting philosopher.
ronsatspokane
Gold $$ Contributor
Not so sure about that. Any distortion of the light entering the objective lens will impact the image in strange and wondrous ways. Finding out whether it is chromatic or non-chromatic would require some lab instruments that most of us don't own or test beds and pattern generators that most of us also don't have. Some can easily be seen in the resulting image, others are more difficult and when you have multiple aberrations, separating them into their component parts can be tricky. Depending on severity and type of course.
The company I currently work for is a job shop so we make whatever our customers need, except eye glasses/contacts, but I can't generally talk about specific customers. But our biggest customer base is medical and military contractors. So we end making optics that do anything from going up into space or going up into your butt! I'm in charge of the department that makes all of the aspherical optics as well as other challenging things such as very high precision lenses/flats and certain metrology tools. My personal specialty is in CNC controlled sub-aperture tools for grinding and polishing. Sometimes I have no idea what the lenses I am making go into, I just know what they are supposed to look like! But I certainly do a lot of optics for the government contractors for sighting and surveillance as well as industrial optics such as condenser lenses (the aspherical optics give more even illumination) and laser applications (removing the spherical aberrations with aspheres allows for more energy where it is needed in the beam for cutting or other things). In my past work I traveled all over the world training people on how to polish precision optics with sub-aperture tools with a large customer base making very large lenses out of calcium flouride or very pure fused silica for micro lithography (making computer chips). Those were some expensive optics!
I lived in Rochester, NY for about 14 years after college and there you meet tons of people who make optics for a living (it's where Kodak, Xerox, and B&L got their start not to mention hundreds of spin-offs)! Some really interesting stuff, and people, there...
Justin
Turbulent Turtle
F-TR competitor
I'm going to catch hell for this, but you talk about "calcium flouride." I'm betting you meant calcium "fluoride". There is a difference between flour and fluor. 
It's such a common mistake, but it hits me like a ton of, well, bricks. I also dabble in photography fora (the proper plural form of forum) and other similar dens of iniquity. I pointed out the same mistake made by a high-end photography guru. He didn't take it well. Divas, what can you do?
However, the rest of your post was very interesting.
BTW, isn't Rochester ground zero for nuclear Armageddon according to Countersniper scopes according to their propaganda?
It's such a common mistake, but it hits me like a ton of, well, bricks. I also dabble in photography fora (the proper plural form of forum) and other similar dens of iniquity. I pointed out the same mistake made by a high-end photography guru. He didn't take it well. Divas, what can you do?However, the rest of your post was very interesting.
BTW, isn't Rochester ground zero for nuclear Armageddon according to Countersniper scopes according to their propaganda?
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