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.
I've been following this newsletter for a while as I have a passing interest in (photographic/cinematic) lens design.
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.
The new designs are very contrasty and sharp wide open, almost across the frame. The older designs need to be stopped down to achieve critical sharpness, and suffer in the corners. Just so much progress in the past 15 years.
> 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.
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).
> 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.
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> 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.
0. https://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relatio...
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.
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
The higher the Abbe number the lower the dispersion of a material.
Fluorite is ~95
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...
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.