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 p.1 #17 · What does fluorite actually do? | |
Fred Lindsey wrote:
On that catalog from Ohara, is Fluorite S-FPL51, 52, or 53?
And which one would be the standard optical glass used by Canon, or are you saying this differs and thus you can't give a specific value?
To be honest, I'm having trouble what all that means.
How would you, in as simple English as possible, link the crystalline hexagonal structure of Fluorite to its optical properties?
Fluorite is not hexagonal, it is isometric, which under normal conditions means cubic, although octagonal is common too, and occasionally occurs as dodecahedral shapes (12 surfaces).
The fact that it is isometric makes it useful for optical use: this implies that the refractive index is equal in all directions of the crystal lattice. This is also why glass is used in optics: glass is an (under)cooled liquid and behaves isometric as a result, too, but in this case it is not crystalline at all. This is what the crystalline lattice links to its optical use; it could not be used if it wasn't isometric, as you would not be able to predict or calculate the results from refraction and reflection.
The colour it shows most predominatly is like caused by the size of the crystal lattice, the distance between the atoms in the lattice IOW. Furthermore, it got its name from being fluorescent (or maybe the other way around). By heating fluorite, it becomes fluorescent with a green tinge of light, likely the same band of light you mentioned.
A lens is made of several types of glass with different properties, so it is impossible to say which specific values are used. Each individual piece of glass used in a lens has its own refractive index. This is done in order to eliminate different optical aberrations introduced by the overall lens design and the ones introduced by other lens elements. IOW, a similar reason as to why fluorite is used.
As others mentioned, it has very low dispersion, which means that it doesn't have strongly different refraction indices for different wavelengths of light like f.e. glass. IOW, it doesn't show the rainbow spectrum as clearly or strongly as you get with white light refracted by a glass prism. This is very advantageous for lens design, as the different wavelengths of light will focus more closely at the focal point of the lens than with ordinary glass, or even most optical glass types.
A disadvantage is that the refractive index is quite low, which means that its use is somewhat limited. Long lenses or telelenses benefit the most in this regard, but then, they need it most because the focusing points of different wavelengths due to the large FLs show here the largest separation in absolute terms, and hence the least convergence, resulting in less sharp images. Fluorite helps prevent this.
Fluorite has a few disadvantages, as it is inherently less stable than glass elements, due to humidity f.e; as a result it needs to be sealed in, which is easier to achieve in long lenses, with a lot of space, rather than in WA lenses with very little space and a lot of elements. Furthermore, it expands and contracts much more than glass, changing the optical properties under extreme temperature variations, be it temporarily. This is supposedly the reason why Canon's long lenses are white or cream rather than black: white reflects heat and light, black absorbs it, contributing to higher temperature fluctuations in a black barrel compared to a barrel painted white (or cream).
Although it would probably be possible to create aspherical lens elements from fluorite, I suspect that due to fluorite's relative softness and brittleness, this is very, very difficult, and hence the reason why we haven't seen any Canon lenses with aspherical fluorite lenses so far. I don't expect this to change. Growing the crystals required to make lens elements from, takes 1 to 2 years, and making a spherical lens is a major operation in the first place, let alone an aspherical one. I reckon it would be 10X as expensive, if not more, to make an aspherical lens element from fluorite than it is to make a spherical one, and it would have to be made like a precision aspherical lens; it can't be moulded for starters (it is a crystal, not an undercooled liquid), and the grinding and polishing process would be a lot slower than with glass aspherical lenses to prevent breaking, splitting, etc.
For fluorite to be used in optics, it has to be optically pure, and this is something you don't find in nature, normally. In the past, some lens elements used in (expensive) microscopes were made from naturally occurring fluorite, but these generally are very, very small, nothing like the big plates used in some of the Canon lenses.
AFAIK, Canon grows and grinds their own fluorite, and I don't think they have outsourced this part of the operation to a third party optical glass manufacturer. Also AFAIK, Canon is the only company in the world being able to grow the very large crystals required for their telelenses, and I do think they will keep their secret of success a secret for a long time to come.
Anyway, HTH, kind regards, Wim
(former geologist, with a little, very little, knowledge of mineralogy and crystallography)
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That is a social/economic issue I will write about in my piece don't worry about that. 
