I still haven't come across a summary I liked so I'll try:
Refractive index of a material, typically ~1.5, is not a fixed single number for material. Rather it is wavelength dependent, because diffraction is of course quantum interference thing strengthening at new directions and canceling out elsewhere. Wavelength-index plot shows some sort of exponential or asymptotic, monotonically decreasing curve from UV towards IR.
This means any convex lens always has a higher than intended magnification at blue, higher still at green, okay at red, and only technically right at Sodium vapor yellow, creating "aberrated(NOT after Ernst Abbe)" color-shifted image at its focal point.
To counter this, convex and concave lenses built from different chemical compositions that show different rates of decreasing indices are used, such as Schott BK7 and F2, so that extra positive magnification for blue at first convex lens cancels out with extra negative power for blue at following concave lens, and so on. The chain of lenses can be continued to cancel out effects at as many additional wavelengths, as well as side effects and other types of imperfections, as desired.
Significance of Fluorite or CaF2 crystals in this context is, this material shows a completely flat curve on that wavelength-refractive index plot, referred to as "abnormal dispersion". It naturally focuses all colors across visible spectrum to a same point, skipping over a lot of lens and lens canceling out. Challenge is scaling out camera-sized crystals of Calcium and Fluoride with optical clarity is hard, which Canon has been trying for a few decades.
It would be interesting to know if the term was or wasn't related.
I've been around enough brainstorming sessions to see people come up with sneaky ways to name things after themselves; someone named Abbe deciding to use "aberration" to describe the particular distortion of an image because it sounds like Abbe is totally plausible.
On the other hand, if the term predated Abbe's work and the creation of the Abbe number, it's also possible Abbe decided to work on the problem -- or his mentor assigned him the topic -- because Abbe sounds like aberration.
(It doesn't mean there is a connection, I'm just saying that just because the etymology of the word is independent doesn't mean the use of the word is also.)
> Challenge is scaling out camera-sized crystals of Calcium and Fluoride with optical clarity is hard, which Canon has been trying for a few decades
Not really an informative summary on that. They’ve been succeeding, not trying, for decades. The problem of growing the crystals was solved in the 60s and this is commonplace now.
Fluorite is also used in Fuji lenses, and Nikon/Sony have their own special glass to deal with the same problems.
Yeah thank you, that summary is better than the article.
The definition of refracitve index in the article is also just wrong, since it is simply not an angle. It can be calculated from incidence and refraction angles of the light beam - very different. See https://en.m.wikipedia.org/wiki/Snell%27s_law
To add to your answer, the refractive index is not just wavelength dependent, but can also be depending on the polarization of light, leading to birefringence: https://en.m.wikipedia.org/wiki/Birefringence
For HNers and CompSci people, optics is a notoriously difficult field and much more frustrating.
If you break up the Nobel Prizes a bit differently, then the filed of Optics becomes the most dominant. So very many breakthroughs in science are because of some new optics method. Mostly in the bio/chem fields, it's about gaining a new form of 'contrast' (very broadly defined).
People have spent decades trying to align some little crystal just the right way. Or they did it in their living room with cardboard in a weekend. It's a frustrating field.
One fun thing to remember about lenses are that they aren't really light bending thingys, but more accurately a lens is a Fourier transformer. Of a sort. Again, optics s frustrating.
One fun thing for the more matrix-ly minded are Mueller Matrices. Most modern optics SW is based on this calculus, though it goes a lot further nowadays. Also, most developments in optics are all about the little exceptions that Mueller matrices have.
'naturally focuses all colors across visible spectrum to a same point' would be no dispersion, not 'abnormal dispersion'. abnormal dispersion (usually called anomalous dispersion) is when the refractive index increases with increasing wavelength, instead of decreasing as in normal dispersion
if you had a material with no dispersion you could just make a lens out of it and avoid chromatic aberration, but since you don't, you need to use the dispersions of different materials to cancel it out in the way you describe
fluorite doesn't have anomalous dispersion in the visible spectrum, it just has low dispersion
canon has evidently successfully been scaling out camera-sized crystals of calcium fluoride since the 01960s. other companies have too actually; https://en.wikipedia.org/wiki/Fluorite says
> In the laboratory, calcium fluoride is commonly used as a window material for both infrared and ultraviolet wavelengths, since it is transparent in these regions (about 0.15 µm to 9 µm) and exhibits an extremely low change in refractive index with wavelength. Furthermore, the material is attacked by few reagents. At wavelengths as short as 157 nm, a common wavelength used for semiconductor stepper manufacture for integrated circuit lithography, the refractive index of calcium fluoride shows some non-linearity at high power densities, which has inhibited its use for this purpose. In the early years of the 21st century, the stepper market for calcium fluoride collapsed, and many large manufacturing facilities have been closed. Canon and other manufacturers have used synthetically grown crystals of calcium fluoride components in lenses to aid apochromatic design, and to reduce light dispersion. This use has largely been superseded by newer glasses and computer-aided design. As an infrared optical material, calcium fluoride is widely available and was sometimes known by the Eastman Kodak trademarked name "Irtran-3", although this designation is obsolete.
sodium, fluorite, calcium, and fluoride are not brand names or other proper nouns and thus should not be capitalized in english as they are in german
Abbe was amazing. He worked with Zeiss and a couple other glass manufacturers to systematize optics. One of his greatest accomplishments, beyond levelling up glass quality, was developing an actual "theory of optics" which explained optical phenomena in terms of diffraction. He defined diffraction limited imaging, which meant that people were able to resolve details as fine as opticals can possibly allow (this has only recently been surpassed using Nobel-prize-winning technology). Abbe illumination, which is a way to set up your microscope's light paths to get optimal quality.
He is also known for introducing the eight hour workday(!) and all sorts of employee/company innovations.
My friend from grad school shows how to set up abbe illumination and talks/shows a bit about how to set up optical fourier transforms.
https://www.youtube.com/watch?v=d8Tqoo0S6gc
If you want to draw a straight line of technology development that led to industrialization and an incredible increase in life quality, it goes right through Abbe (and Newton, Pasteur, Maudsley, and Rutherford). All of these people were absolute giants who saw far past the limitations of their day and continue to inspire new generations of geniuses who can take advantage of the amazing resources we have available today (thorlabs.com is a good example).
in the context of apochromatic lenses: those that are optimized for three different wavelengths of light, not just two, which an achromatic lens does.
In the old days, calculations were done by hand, not that this is a big deal, but the big deal is that some really outstanding lens designs were made this way. Computers make the optimization extremely fast these days, but the calculations rely on the properties of the elements, which also vary in cost, durability and so on. So the computer can’t optimize for a continuous range of refractive indices and dispersions, only discrete real-world ones that the glass makers list, and which are specified (or not) to the program by the designer.
Fluorine also has special properties as a coating.
> Computers make the optimization extremely fast these days...
A lot of computer design is now aimed at optimising for tolerances in lens elements and mechanical housings to reduce precision necessary when assembling them: they can drop elements into the tube and ship them off with little or no calibration: this is done particularly with kit lenses which are price sensitive.
I did error budgeting for environmental effects on optical systems at Itek. Glad to see so many optics folks here. I wonder who they are, but won't ask. ;-)
> in the context of apochromatic lenses: those that are optimized for three different wavelengths of light, not just two, which an achromatic lens does.
Achromatic lenses (corrected for blue and green) was acceptable for black and white orthochromatic film and plates which are only sensitive to blue and green. Even with panchromatic b+w film, achromatic lenses are usually satisfactory, but the chromatic aberration becomes visible with colour film. Aprochromatic lenses are corrected for blue, green and red.
Lenses can also be corrected for broader spectrums that include UV and IR: Nikon made such a lens – the UV 105mm f4.5 – for technical/scientific applications.
Telescopes for astronomy are often corrected well into the infrared regime due to some of the fairly interesting emission lines that can be imaged using an appropriate camera.
I mean, dispersion matters simply because people generally don't want colored fringing and general haziness in their photos?
There are two types of chromatic aberration (CA), both caused by dispersion.
* Axial: perfect focusing is impossible because different wavelengths focus at different distances. If green light is focused correctly, then red and blue light is out of focus. This affects the entire image and is very difficult to fix in post-processing.
* Transverse: there's no unique image because magnification depends on wavelength. Blue light forms a slightly larger image than green, and green larger than red. This manifests as color fringing, most apparent near the edges and corners of the image, far from the optical axis. This can be alleviated algorithmically, by resizing the red and blue sub-images to match the green one.
Future improvements in simulations and manufacturing will probably enable micromanaging light in 3 dimensions. Ive seen some very strange fractal and composite lenses.
Fluorite lenses are incredible, but not a critical win until you get to high mag or up against the speed limits of your system. When it really becomes apparent is when coupled with VR stabilization, which can get you 4+ EV stops of speed. When the subject is that crisp, the chromatic aberration is significant, or, with fluorite, not. Interestingly, modern photography suites offer both correction of effects of lenses (e.g vignetting and pincushion) and chromatic aberration in silicon. Compared to where I started with a circa 1989 Nikkor 70-210 f/4 and film, the pictures I can take today are incredible. The real problem, for me, is editing down to a digestible number of eye-poppingly good shots. I imagine kids these days are thoroughly unimpressed, but I'm humbled by the incredible amount of engineering that has gone into photography.
> ... modern photography suites offer both correction of effects of lenses (e.g vignetting and pincushion) and chromatic aberration...
Yeah. I haven't worried about chromatic aberration in lenses for about 15 years now. It's trivial to automatically correct in post processing, as long as you shoot RAW.
The fluorite lenses also allow them to shrink the lens design and make it easier to carry around. They do this by reducing the # of elements required. It's why you mostly see them in big long focal length lenses since that's where the size & weight gets really obnoxious.
As am I. Starting with film in a similar era and moving along with every 1 or 2 major generational shift in CMOS since then, I’m delighted to be at a point today where my output is bottlenecked by the actual 55000mbps M2 SSD in my MacBookPro for whizzing through hundreds of GB of stills for every shoot to distill down to about 10 best ones.
I have a FD 300/2.8 S.S.C. Fluorite lens, introduced in 1975. It's a FANTASTIC lens, sharp corner to corner wide open at f/2.8, and excellent for astrophotography. It's able to capture details of the Orion nebula, horsehead nebula, and the spiral arms of the Andromeda galaxy in a single shot. It's also excellent for outdoor portraits and creams backgrounds, though you'll need to use a phone call on speakerphone to talk to your subject.
The list price back then was 420000 JPY which is $5364 in today's dollars. I got it for $400, used.
Back then prices of FD mount lenses dropped dramatically because nobody wanted them: the flange distance was too short to be adapted to any DSLR. I ended up taking apart the entire back part and machining a conversion mount to make it usable on DSLR. (An adapter ring won't work, since it adds thickness.)
Unfortunately with the advent of mirrorless cameras, FD lenses are once again usable with simple adapter rings, and their used market prices have gone back up. However they're still excellent, excellent value for $ in comparison to modern autofocusing equivalents in optics; the lens I have costs ~$600-$1000 on eBay now whereas a new Sony 300/2.8 GM costs $6000. For anyone looking for a fast, large aperture telephoto lens I'd highly recommend looking into FD lenses that have fluorite elements, as long as you don't mind manual focus.
I used to have a FD 500mm f/4.5 which is nice but not as well corrected as modern glass. I got it for $750 which, too, is a bargain.
> as long as you don't mind manual focus
Given that the primary use case of large aperture telephoto lenses is sports and wildlife, fast autofocus is a killer feature. Moreover, modern computer optimization has managed to vastly lighten the weight and improve the weight distribution of the lens, not to mention impeccable image quality, which is why for many the $6000 is more than justified.
I wasn't aware that Canon used flourite elements. You learn something new every day. Nice. Their telephoto lenses also use holographic elements, and I attended a talk (1990 ish) where one of their lens designers spoke about a clever scheme using diffraction order pairs (n and n + 1) to compensate each other. This allowed them to use diffractive dispersion in addition to glass (and now I know flourite) for color correction without introducing stray light (ghosts) due to unwanted diffraction orders, which are nearly impossible to get rid of. These lenses are works of art.
Some Canon lenses like the EF 17-40mm L use "replica" aspherical elements...
Replica aspherical lens elements are produced by using an aspherical surface mold and ultraviolet-light-hardening resin to form an aspherical surface layer on a spherical glass lens.
I was kind of hoping they would explain why it helps prevent chromatic aberration. Unfortunately their explanation stops short, basically just saying that 'it does' without going into the details of why.
My first guess would be something to do with it having a well suited refractive index, but it is almost equal to that of glass. The best candidate I've found is that the group velocity dispersions are opposite, which seems like it might explain it, if only I knew what it meant.
When a lens is designed, typically an optimiser is used to try to find a combination of lens elements that focuses light of all wavelengths to the same spot across the whole image. If all the same type of glass is used, then it's hard for the optimiser to find a solution that corrects for the chromatic aberration across all the spectrum, because if it adjusts (for instance) something to fix the green aberration then it'll mess up the blue and vice-versa. But if a different type of glass is used such as fluorite, which has a different pattern of chromatic dispersion, then that gives the optimiser an extra degree of freedom, so it is more able to independently control the aberration in the different colours and make a lens that performs well.
Over the past ten years, lenses have gotten unbelievably better. Apsherical elements have done some of the lifting, and glass formulas another, but how much have computational tools changed how lenses are designed?
It's a bit buried, but this (to me) is the most interesting sentence :
> Fluorite lenses are also unique in their extraordinary partial dispersion tendencies: the red to green wavelengths are dispersed with the same tendencies as glass, but the green to blue wavelengths are dispersed more than glass.
It has both low dispersion (less overall aberrations) and a unique shape to the dispersion curve (more control). Glasses typically all have similar dispersion curves. The weird shape of fluorite's dispersion curve gives your optimization function an extra lever to play with.
Does this mean it has an index of refraction that depends on wavelength? How can that be? Is the bond length some multiple of blue (or green/red), where there's some quick "change" in what the photon "sees"?
This article shows a "map" of available glasses, with each glass shown by its refractive index and dispersion constant -- in a particular way. The explanation is that if all you have are glasses on that "glass line," you can make a lens with equal focal length at two wavelengths but not three. You need glasses that are not on the "glass line," and one of them is calcium fluoride. Also, some plastics such as acrylic and polystyrene can be used, but have their own issues such as thermal expansion.
Finding a lens with 2 equal focal lengths is equivalent to a graph of focal length versus wavelength that's roughly a parabola. With 3 equal focal length, the graph looks like a cubic curve.
The glasses also have to be economical, able to take a good polish, clear, chemically resistant (to avoid staining), mechanically robust, etc. It's not an easy design problem since the "exotic" glasses with extreme refractive properties also tend to have worse properties overall.
I found the article explained exactly why using fluorite helped chromatic aberration. It adds another degree of freedom, due to the partial dispersion.
'the red to green wavelengths are dispersed with the same tendencies as glass, but the green to blue wavelengths are dispersed more than glass. Using a convex fluorite lens element alongside a high-dispersion glass concave lens element therefore eliminates residual chromatic aberration'
I thought the reason underlying that is that fluorite is not a glass, technically. It is a crystal. But I’m not a MatSci person so that doesn’t leave me any bit more informed.
Possibly a poor analogy, but imagine making a particle accelerator with the goal that objects of different mass put into it are accelerated and leave it going the same velocity.
The current state of technology allows that the best we can do is make one that accelerates lighter objects slightly more than heavier objects.
However, we've found a way to make an accelerator using a different design that accelerates heavier objects more than lighter objects. If we pass objects through the first accelerator, then the second, the second accelerator reverses out some of the non-linearity of the first. Unfortunately this second accelerator is very, very expensive to make.
This is how lenses of different refractive indexes are used: one element partially corrects the dispersion produced by earlier elements. Fluorite has the right refractive index to correct aberrations created in long focal length lenses.
Chromatic aberration happens because of dispersion --- the fact that the refractive index changes with wavelength. By combining materials with different properties (e.g. low dispersion, or high refractive index), you can cancel out the aberration in lens designs such as achromatic doublets, apochromatic triplets, etc.
In general, materials that are high in refractive index have high dispersion (e.g. crown glass) and materials that have low dispersion also have low refractive index. But ideally we want a material that has high refractive index but low dispersion so that it can both bend light without introducing a lot of chromatic aberration.
Fluorite has extraordinarily low dispersion while having a refractive index that's only slightly lower than glass, making it a good material to be used in conjunction with other materials.
> Fluorite just has low dispersion and therefore a lens made from it has less chromatic aberration than glass.
So why not make the entire lens from fluorite? Because pairing an element with one with different refractive index can lower the overall dispersion.
Typically you'll see compound lenses are made of pairs of elements where one is positive (convex) and the other negative (concave): instead of making one lens with power of, say, +4, the group is made from one that's +5 and one that's -1 of different type of glass so the refractive indexes cancel-out dispersion and other aberrations.
Readit News
Refractive index of a material, typically ~1.5, is not a fixed single number for material. Rather it is wavelength dependent, because diffraction is of course quantum interference thing strengthening at new directions and canceling out elsewhere. Wavelength-index plot shows some sort of exponential or asymptotic, monotonically decreasing curve from UV towards IR.
This means any convex lens always has a higher than intended magnification at blue, higher still at green, okay at red, and only technically right at Sodium vapor yellow, creating "aberrated(NOT after Ernst Abbe)" color-shifted image at its focal point.
To counter this, convex and concave lenses built from different chemical compositions that show different rates of decreasing indices are used, such as Schott BK7 and F2, so that extra positive magnification for blue at first convex lens cancels out with extra negative power for blue at following concave lens, and so on. The chain of lenses can be continued to cancel out effects at as many additional wavelengths, as well as side effects and other types of imperfections, as desired.
Significance of Fluorite or CaF2 crystals in this context is, this material shows a completely flat curve on that wavelength-refractive index plot, referred to as "abnormal dispersion". It naturally focuses all colors across visible spectrum to a same point, skipping over a lot of lens and lens canceling out. Challenge is scaling out camera-sized crystals of Calcium and Fluoride with optical clarity is hard, which Canon has been trying for a few decades.
This is a clever bit of folk etymology [1], but aberrate is derived from the Latin verb aberro, meaning to wander or stray [2].
[1]: https://en.wikipedia.org/wiki/Folk_etymology
[2]: https://en.wiktionary.org/wiki/aberro#Latin
I've been around enough brainstorming sessions to see people come up with sneaky ways to name things after themselves; someone named Abbe deciding to use "aberration" to describe the particular distortion of an image because it sounds like Abbe is totally plausible.
On the other hand, if the term predated Abbe's work and the creation of the Abbe number, it's also possible Abbe decided to work on the problem -- or his mentor assigned him the topic -- because Abbe sounds like aberration.
(It doesn't mean there is a connection, I'm just saying that just because the etymology of the word is independent doesn't mean the use of the word is also.)
Not really an informative summary on that. They’ve been succeeding, not trying, for decades. The problem of growing the crystals was solved in the 60s and this is commonplace now.
Fluorite is also used in Fuji lenses, and Nikon/Sony have their own special glass to deal with the same problems.
The definition of refracitve index in the article is also just wrong, since it is simply not an angle. It can be calculated from incidence and refraction angles of the light beam - very different. See https://en.m.wikipedia.org/wiki/Snell%27s_law
To add to your answer, the refractive index is not just wavelength dependent, but can also be depending on the polarization of light, leading to birefringence: https://en.m.wikipedia.org/wiki/Birefringence
If you break up the Nobel Prizes a bit differently, then the filed of Optics becomes the most dominant. So very many breakthroughs in science are because of some new optics method. Mostly in the bio/chem fields, it's about gaining a new form of 'contrast' (very broadly defined).
People have spent decades trying to align some little crystal just the right way. Or they did it in their living room with cardboard in a weekend. It's a frustrating field.
One fun thing to remember about lenses are that they aren't really light bending thingys, but more accurately a lens is a Fourier transformer. Of a sort. Again, optics s frustrating.
One fun thing for the more matrix-ly minded are Mueller Matrices. Most modern optics SW is based on this calculus, though it goes a lot further nowadays. Also, most developments in optics are all about the little exceptions that Mueller matrices have.
Still, a good little thing to read about, if interested: https://en.wikipedia.org/wiki/Mueller_calculus
Are you saying the word aberration comes from Ernst Abbe's last name? Because it doesn't, it comes from latin. https://www.etymonline.com/word/aberration
'naturally focuses all colors across visible spectrum to a same point' would be no dispersion, not 'abnormal dispersion'. abnormal dispersion (usually called anomalous dispersion) is when the refractive index increases with increasing wavelength, instead of decreasing as in normal dispersion
https://en.wikipedia.org/wiki/Dispersion_(optics)#Material_d...
if you had a material with no dispersion you could just make a lens out of it and avoid chromatic aberration, but since you don't, you need to use the dispersions of different materials to cancel it out in the way you describe
fluorite doesn't have anomalous dispersion in the visible spectrum, it just has low dispersion
canon has evidently successfully been scaling out camera-sized crystals of calcium fluoride since the 01960s. other companies have too actually; https://en.wikipedia.org/wiki/Fluorite says
> In the laboratory, calcium fluoride is commonly used as a window material for both infrared and ultraviolet wavelengths, since it is transparent in these regions (about 0.15 µm to 9 µm) and exhibits an extremely low change in refractive index with wavelength. Furthermore, the material is attacked by few reagents. At wavelengths as short as 157 nm, a common wavelength used for semiconductor stepper manufacture for integrated circuit lithography, the refractive index of calcium fluoride shows some non-linearity at high power densities, which has inhibited its use for this purpose. In the early years of the 21st century, the stepper market for calcium fluoride collapsed, and many large manufacturing facilities have been closed. Canon and other manufacturers have used synthetically grown crystals of calcium fluoride components in lenses to aid apochromatic design, and to reduce light dispersion. This use has largely been superseded by newer glasses and computer-aided design. As an infrared optical material, calcium fluoride is widely available and was sometimes known by the Eastman Kodak trademarked name "Irtran-3", although this designation is obsolete.
sodium, fluorite, calcium, and fluoride are not brand names or other proper nouns and thus should not be capitalized in english as they are in german
Since 1136? :p
(sorry, can't have a pedantic thread without pedantic jokes, it's obligatory)
He is also known for introducing the eight hour workday(!) and all sorts of employee/company innovations.
My friend from grad school shows how to set up abbe illumination and talks/shows a bit about how to set up optical fourier transforms. https://www.youtube.com/watch?v=d8Tqoo0S6gc
If you want to draw a straight line of technology development that led to industrialization and an incredible increase in life quality, it goes right through Abbe (and Newton, Pasteur, Maudsley, and Rutherford). All of these people were absolute giants who saw far past the limitations of their day and continue to inspire new generations of geniuses who can take advantage of the amazing resources we have available today (thorlabs.com is a good example).
https://www.targettamers.com/guides/apochromatic-lenses/
in the context of apochromatic lenses: those that are optimized for three different wavelengths of light, not just two, which an achromatic lens does.
In the old days, calculations were done by hand, not that this is a big deal, but the big deal is that some really outstanding lens designs were made this way. Computers make the optimization extremely fast these days, but the calculations rely on the properties of the elements, which also vary in cost, durability and so on. So the computer can’t optimize for a continuous range of refractive indices and dispersions, only discrete real-world ones that the glass makers list, and which are specified (or not) to the program by the designer.
Fluorine also has special properties as a coating.
https://www.digitalcameraworld.com/features/this-is-why-your...
A lot of computer design is now aimed at optimising for tolerances in lens elements and mechanical housings to reduce precision necessary when assembling them: they can drop elements into the tube and ship them off with little or no calibration: this is done particularly with kit lenses which are price sensitive.
Achromatic lenses (corrected for blue and green) was acceptable for black and white orthochromatic film and plates which are only sensitive to blue and green. Even with panchromatic b+w film, achromatic lenses are usually satisfactory, but the chromatic aberration becomes visible with colour film. Aprochromatic lenses are corrected for blue, green and red.
Lenses can also be corrected for broader spectrums that include UV and IR: Nikon made such a lens – the UV 105mm f4.5 – for technical/scientific applications.
There are two types of chromatic aberration (CA), both caused by dispersion.
* Axial: perfect focusing is impossible because different wavelengths focus at different distances. If green light is focused correctly, then red and blue light is out of focus. This affects the entire image and is very difficult to fix in post-processing.
* Transverse: there's no unique image because magnification depends on wavelength. Blue light forms a slightly larger image than green, and green larger than red. This manifests as color fringing, most apparent near the edges and corners of the image, far from the optical axis. This can be alleviated algorithmically, by resizing the red and blue sub-images to match the green one.
Yeah. I haven't worried about chromatic aberration in lenses for about 15 years now. It's trivial to automatically correct in post processing, as long as you shoot RAW.
The list price back then was 420000 JPY which is $5364 in today's dollars. I got it for $400, used.
Back then prices of FD mount lenses dropped dramatically because nobody wanted them: the flange distance was too short to be adapted to any DSLR. I ended up taking apart the entire back part and machining a conversion mount to make it usable on DSLR. (An adapter ring won't work, since it adds thickness.)
Unfortunately with the advent of mirrorless cameras, FD lenses are once again usable with simple adapter rings, and their used market prices have gone back up. However they're still excellent, excellent value for $ in comparison to modern autofocusing equivalents in optics; the lens I have costs ~$600-$1000 on eBay now whereas a new Sony 300/2.8 GM costs $6000. For anyone looking for a fast, large aperture telephoto lens I'd highly recommend looking into FD lenses that have fluorite elements, as long as you don't mind manual focus.
> as long as you don't mind manual focus
Given that the primary use case of large aperture telephoto lenses is sports and wildlife, fast autofocus is a killer feature. Moreover, modern computer optimization has managed to vastly lighten the weight and improve the weight distribution of the lens, not to mention impeccable image quality, which is why for many the $6000 is more than justified.
For wildlife, manual focus is actually not that hard with some practice, as long as it's not birds.
For sports, yeah, it's difficult.
Here's a video by Gordon Laing showing a "Canon lens TEARDOWN! What's INSIDE a new lens?"
https://youtube.com/watch?v=YH5_nVRWHZ0
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Replica aspherical lens elements are produced by using an aspherical surface mold and ultraviolet-light-hardening resin to form an aspherical surface layer on a spherical glass lens.
My first guess would be something to do with it having a well suited refractive index, but it is almost equal to that of glass. The best candidate I've found is that the group velocity dispersions are opposite, which seems like it might explain it, if only I knew what it meant.
> Fluorite lenses are also unique in their extraordinary partial dispersion tendencies: the red to green wavelengths are dispersed with the same tendencies as glass, but the green to blue wavelengths are dispersed more than glass.
It has both low dispersion (less overall aberrations) and a unique shape to the dispersion curve (more control). Glasses typically all have similar dispersion curves. The weird shape of fluorite's dispersion curve gives your optimization function an extra lever to play with.
Finding a lens with 2 equal focal lengths is equivalent to a graph of focal length versus wavelength that's roughly a parabola. With 3 equal focal length, the graph looks like a cubic curve.
https://www.opticsforhire.com/blog/apochromatic-lens/
The glasses also have to be economical, able to take a good polish, clear, chemically resistant (to avoid staining), mechanically robust, etc. It's not an easy design problem since the "exotic" glasses with extreme refractive properties also tend to have worse properties overall.
'the red to green wavelengths are dispersed with the same tendencies as glass, but the green to blue wavelengths are dispersed more than glass. Using a convex fluorite lens element alongside a high-dispersion glass concave lens element therefore eliminates residual chromatic aberration'
The current state of technology allows that the best we can do is make one that accelerates lighter objects slightly more than heavier objects.
However, we've found a way to make an accelerator using a different design that accelerates heavier objects more than lighter objects. If we pass objects through the first accelerator, then the second, the second accelerator reverses out some of the non-linearity of the first. Unfortunately this second accelerator is very, very expensive to make.
This is how lenses of different refractive indexes are used: one element partially corrects the dispersion produced by earlier elements. Fluorite has the right refractive index to correct aberrations created in long focal length lenses.
In general, materials that are high in refractive index have high dispersion (e.g. crown glass) and materials that have low dispersion also have low refractive index. But ideally we want a material that has high refractive index but low dispersion so that it can both bend light without introducing a lot of chromatic aberration.
Fluorite has extraordinarily low dispersion while having a refractive index that's only slightly lower than glass, making it a good material to be used in conjunction with other materials.
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So why not make the entire lens from fluorite? Because pairing an element with one with different refractive index can lower the overall dispersion.
Typically you'll see compound lenses are made of pairs of elements where one is positive (convex) and the other negative (concave): instead of making one lens with power of, say, +4, the group is made from one that's +5 and one that's -1 of different type of glass so the refractive indexes cancel-out dispersion and other aberrations.
Fun further reading: https://www.canon-europe.com/pro/infobank/fluorite-aspherica...