12 ms·
Fluorite lenses: Corrective capabilities beyond ordinary optical glass
- aj7 3y agoFluorite is an obsolete term in the optical engineering community. The term used today is simply “calcium fluoride.” Calcium fluoride windows are used extensively as the expendable windows of the 308nm XeCl laser, which anneals every thin film display made.
- contravariant 3y agoI 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.
- mnw21cam 3y agoWhen 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.
- postmodest 3y agoOver 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?
- buildbot 3y agoAnecdotally, a lot. Modeling of Aspherical elements has gotten better and it is much easier to make them as well. Mamiya made some of the first APO computer corrected lenses back in 1988 - https://lens-db.com/mamiya-apo-sekor-z-350mm-f56-1988/ https://lens-db.com/mamiya-apo-sekor-z-350mm-f56-1988/
- fsh 3y agoWhat are you basing this observation on? I haven't seen any significant improvement in photography or cinema lens performance over many decades. The principles have been understood for 150 years, and using ray-tracing software is standard since the 70s. Cheap lens systems in phones got a lot better, and digital sensors are much more sensitive than film though.
- buster3000 3y agoNot who you're replying to, but it seems this is a classic case of mistaking not noticing change is evidence of no change. I've been following this newsletter for a while as I have a passing interest in (photographic/cinematic) lens design. https://www.pencilofrays.com/ https://www.pencilofrays.com/
- mnw21cam 3y agoThe main reason why lenses have become significantly better over the last 10-20 years (certainly at the consumer level) is mainly because camera sensors have become significantly better over the last 10-20 years, and therefore the lenses have had to be engineered to a higher standard to avoid the lens foibles being quite so obvious in the higher resolution images that are now possible. The engineering principles are largely the same with a few smaller advances, but to accomodate the demanded-for improvements consumer lenses are now bigger, heavier, more expensive, and have more interesting optical designs. It's purely demand-driven, not technology-driven.
- ben7799 3y agoExactly. It can't be ignored that these new lenses are much bigger & heavier and much more expensive as a result. That helps a lot. But it makes them worse if a) you need to carry them a long way for your type of photography b) your photography doesn't actually benefit from or require their advantages. A lot of these benefits seem more important to the measurement focused hobbyists. But if you don't actually care about shooting at f/1.2-f/1.4 a lot of the time you might want to save the money.
- deleted 3y ago[deleted]
- buildbot 3y agoI found the article explained exactly why using fluorite helped chromatic aberration. It adds another degree of freedom, due to the partial dispersion.
- NickNameNick 3y agoIsn't that covered in the 7th paragraph? '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'
- lambdasquirrel 3y agoI 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.
- rainbowzootsuit 3y agoFluorite just has low dispersion and therefore a lens made from it has less chromatic aberration than glass.
- davidmurdoch 3y ago> the red to green wavelengths are dispersed with the same tendencies as glass, but the green to blue wavelengths are dispersed more than glass.
- rainbowzootsuit 3y agoI admittedly responded without reading the source, so please accept my apologies. Despite how it is worded the total dispersion of Fluorite is less than glass. That quote is referring to the ‘extraordinary partial dispersion’ property. They are using this to better correct the aberration than could be done with two pieces of glass alone. This seems to be illustrated in the diagrams well. Some further stuff to read up on is the Abbe number that describes the refractive index vs wavelength derived from a set of light sources. https://en.wikipedia.org/wiki/Abbe_number https://en.wikipedia.org/wiki/Abbe_number The higher the Abbe number the lower the dispersion of a material. Fluorite is ~95 https://refractiveindex.info/?shelf=main&book=CaF2&page=Malitson https://refractiveindex.info/?shelf=main&book=CaF2&page=Mali... Random choice below but glass tends to be ~25 - 80 https://refractiveindex.info/?shelf=glass&book=HOYA-C&page=E-C3 https://refractiveindex.info/?shelf=glass&book=HOYA-C&page=E...
- foobar1962 3y ago> 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.
- deleted 3y ago[deleted]
- itishappy 3y agoIt'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.
- nomel 3y agoDoes 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"?
- dekhn 3y agoGlass has this too. it's why achromats and apochromats were developed (using rather clever ideas like pairing two different types of glass, which cancel each other's dispersions. All the physical explanations I've seen invoke the EM equations and quantum mechanics to explain it (and I don't understand it well enough to translate).
- aoeusnth1 3y ago3Blue1Brown has a nice animated video about this: https://youtu.be/KTzGBJPuJwM?si=ssBPKL8LVhsQ3wNZ https://youtu.be/KTzGBJPuJwM?si=ssBPKL8LVhsQ3wNZ
- tonyarkles 3y agoThis is, I think, just one of those spots where "all models are wrong, some models are useful" is true. From the wikipedia page on Refractive Index (https://en.wikipedia.org/wiki/Refractive_index https://en.wikipedia.org/wiki/Refractive_index): > The refractive index may vary with wavelength. This causes white light to split into constituent colors when refracted. This is called dispersion. This effect can be observed in prisms and rainbows, and as chromatic aberration in lenses. --- snip --- > For most materials the refractive index changes with wavelength by several percent across the visible spectrum. Nevertheless, refractive indices for materials are commonly reported using a single value for n, typically measured at 633 nm. So we were all lied to in our introductory optics classes. n isn't a constant for a given material but rather n(lambda) but weakly.
- deleted 3y ago[deleted]
- s0rce 3y agoIt seemed to, the dispersion is different so you can cancel the dispersion from glass (dispersion is the refractive index variation with wavelength).
- porphyra 3y agoChromatic 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.
- analog31 3y agoThis 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. https://www.opticsforhire.com/blog/apochromatic-lens/ 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.
- foobar1962 3y agoPossibly 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.
- dheera 3y agoI 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.
- porphyra 3y agoI 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.
- dheera 3y agoFor astrophotography manual focus is preferable. 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.
- jcynix 3y agoRecently I learned that today's lenses don't contain "just glass" as optical elements but often specially designed plastic elements too. 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 https://youtube.com/watch?v=YH5_nVRWHZ0
- deleted 3y ago[deleted]
- rodgerd 3y agoIf you like teardowns, I will plug the Lens Rentals blog @ https://www.lensrentals.com/blog https://www.lensrentals.com/blog which features teardowns such as https://www.lensrentals.com/blog/2021/01/the-secret-of-the-broken-element-a-canon-rf-100-500mm-f4-7-7-1-teardown/ https://www.lensrentals.com/blog/2021/01/the-secret-of-the-b...
- porphyra 3y agoToo bad Roger Cicala stopped writing.
- foobar1962 3y agoSome 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.
- kridsdale1 3y agoIsn’t that a super budget lens? How’s the pixel peeper performance?
- matsur 3y ago"L" has a red ring and everything!
- 3y ago
- k310 3y agoI looked for a simple explanation of why dispersion matters, and this seems helpful: https://www.targettamers.com/guides/apochromatic-lenses/ 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-camera-lens-needs-a-fluorine-coating https://www.digitalcameraworld.com/features/this-is-why-your...
- lobochrome 3y agoThe linked article perfectly explains it though no?
- readyplayernull 3y agoNever bother someone who has just came out of a rabbit hole.
- drittich 3y agoI laughed at the fact that you seem so familiar with this state and are sympathetic to it.
- Sharlin 3y agoI 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.
- neom 3y agoSuper UD and UD have been some of the gold standards in lenses for a long time. Fluorite plays a huge part in it. Fun further reading: https://www.canon-europe.com/pro/infobank/fluorite-aspherical-ud-lenses/ https://www.canon-europe.com/pro/infobank/fluorite-aspherica...
- s0rce 3y agoFluorite (CaF2) lenses and windows are also used for infrared applications as they transmit much better than glass.
- DarkSucker 3y agoI 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.
- porphyra 3y agoThe fragility of fluorite was one a reason why Nikon gear was chosen for the International Space Station iirc.
- kridsdale1 3y agoPlus that little red marquee looks rad against the infinite universe.
- helij 3y agoTheir fluorite lenses in Takahashi telescopes are a hit among amateur astronomers.
- tenken 3y agoAnd here I thought they meant Eyes Glasses .... Darn.
- jwrallie 3y agoI also thought they meant eyeglasses. I used glasses for a few years and never cared much for chromatic aberration, but once I started to use contacts, going back to glasses makes it very noticeable, specially on the periphery of the lens. Lenses on glasses need to be very high quality to approach the image quality of contacts in terms of distortion.
- orthoxerox 3y agoLast year I got a new prescription and the optometrist convinced me to switch to higher refractive index lenses, because they are thin, light, don't have a subtle yellow hue like CR-39, don't distort the facial features and so on. I agreed and couldn't wear them at all due to insane chromatic aberration. After a few complaints they relented and replaced the lenses with CR-39 and I have been happy ever since.
- tenken 3y agoDo you mean "High Index" lenses? ... I have strong prescriptions of -9.x and -11.x and I have had 1.66 index lenses. I've always noted to doctors light sources move around or float around alot with the lenses. I haven't necessarily noticed a difference with contacts (but I did see sharper with hard contacts) but my eyes cannot tolerate hard or soft lenses. I have tried even higher index lenses, but anything above 1.66 and the distortions and optical aberrations get too annoying ...
- orthoxerox 3y agoYes, high refractive index lenses. -9 and -11 sound like actually good reasons to recommend them, at these levels of myopia lower index glasses should be noticeably heavier. What I couldn't tolerate was specifically chromatic aberration. Every letter in this text box would have a yellow and blue fringe if my head was turned even slightly. And this would happen with every high-contrast border, like someone's t-shirt, a white car or a house against the sky.
- ChrisMarshallNY 3y agoMy mother was a geologist. She had a piece of fluorite that was clear, and she would put it over things, and it would double them (you'd see two of everything). It has some interesting optical characteristics. If I remember, the stone was soft, and sensitive to shock and temperature changes. Probably makes it challenging to work with.
- daniel_reetz 3y agoPretty sure that was calcite - famous for double refraction. https://sciencedemonstrations.fas.harvard.edu/presentations/double-refraction https://sciencedemonstrations.fas.harvard.edu/presentations/...
- ChrisMarshallNY 3y agoYou are correct. I misremembered. Flourite was ... very fluorescent.
- kragen 3y agoas it turns out, pure fluorite has no fluorescence, even though fluorescence is named for fluorite
- ChrisMarshallNY 3y agoWell, it has been a long time (like, 40 years), but I’m pretty sure I remember it being strongly fluorescent. It might have been impurities, though.
- kragen 3y agoyeah, that's normal, which is why it's called fluorescence. pure natural minerals are very rare
- numpad0 3y agoI 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.
- gregschlom 3y ago> aberrated(after Ernst Abbe) 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 https://www.etymonline.com/word/aberration
- numpad0 3y agoMy mistakes. I stand corrected.
- kpozin 3y ago> creating "aberrated(after Ernst Abbe)" color-shifted image at its focal point 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 https://en.wikipedia.org/wiki/Folk_etymology [2]: https://en.wiktionary.org/wiki/aberro#Latin https://en.wiktionary.org/wiki/aberro#Latin
- daguava 3y ago[dead]
- killjoywashere 3y agoFluorite 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.
- kridsdale1 3y agoAs 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.
- vardump 3y ago> ... 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.
- ben7799 3y agoThe 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.
- djtango 3y agoDon't have much to add, but this article was so pleasing to read. Those diagrams with the light dispersion really tickled me - is that spectrum real or is it a render?
- hiyer 3y agoCurious - is this really expensive to make? Why is chromatic aberration even a thing now if we've had this technology since 1969?
- ssnistfajen 3y agoBecause it's not really something that matters for the vast majority of consumer photography equipment. https://istarscopeclub.proboards.com/thread/247/twisting-facts-caf2-fluorite-apos https://istarscopeclub.proboards.com/thread/247/twisting-fac... ^Some 2012 forum post claiming the material cost $2000/kg, although their wording is ambiguous on whether this was the price for the raw material or the finished lens. But based on the manufacturing process described in the source link of this post, it would appear to be a much more cumbersome and complex process than manufacturing regular glass lenses. Fluorite lens are also more fragile which may impact yield %.
- reiichiroh 3y agoMaybe I’ll get thinner than 1.75 lenses some day for my crazy high prescription.
- londons_explore 3y agoI don't understand why chromatic abberation isn't simply fixed in software. Knowing that your red, green, and blue images will have slightly different focus points and sizes seems pretty easy to fix. For a stationary scene, you simply take 3 separate photos - one focussed for each colour, and then rescaled to match each others sizes. For moving scenes, the math is more complex, but you should still be able to fix most of the issues. The only real issue is light that is "yellow" - ie. halfway in wavelength between different colours. The only perfect fix for that is to have each pixel be a spectrometer. Even that seems possible by using variable-bandgap sensors.
- varjag 3y agoYou can't create information that wasn't in the captured image. CA reduces spatial resolution.
- lizknope 3y agoAll of the current RAW image processing software have features to reduce chromatic aberration. Many of them have databases of lenses that match the EXIF data of the lens you used to take the photo to help automatically adjust the image the right amount.