Fluorite Lenses

[bill_meyer1]

I asked this question some time ago on the Q&A forum and it got rejected due to it not being photographically related. I was somewhat surprised, but don't like to argue with the moderater's. I figured I might post it hear and hopefully get some reasonable responses.

 

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My question is this, what exactly is a fluorite lens element, my basic understanding is that it is a crystal (that Canon has learned to "grow") that has excellent light transmission properties. How does this element differ from UD glass and super UD glass? Also, if this type of element is so good, why don't other manufacturer's produce it as well?

[brad_hutcheson]

I don't like making a fool of myself if I can help it, even on my own site, so I looked up what I could to be sure my brain was working. There is a little bit about flourite in the Canon Camera Museum in the lens technical room. It confirmed what I thought. Flourite IS an ultra low dispersion glass. It just happens to be grown from calcium flouride instead of being a traditional silicon based glass. The purpose is to reduce chromatic aberations.

 

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As far as it being better than other UD glass, I don't have a clue, but I would guess that it has some advantages, or Canon wouldn't even bother even if they do claim to have invented (I think) the process of growing the crystal. Why don't other companies use it? They do. They just don't make 35mm camera lenses. It is expensive. Televue makes some high end refractor telescopes, both with and without flourite elements. The ones with flourite are much more expensive, even taking the larger apertures into acount. In other words, other manufacturers probably just don't think the increase in quality is worth the cost in Yen.

[geoffrey_s._kane3]

Instead of fluorite, Nikon uses their ED (<b>E</b>xtra-low

<b>D</b>ispersion) glass. Nikon prefers ED glass to fluorite

because:<br>

1. It's less sensitive to temperature changes --this

supposedly explains why Nikon super-tele lenses are black and Canon

super-tele lenses are white.<br>

2. It's harder and more durable and can be used as the front element

of a lens --many older lenses had ED front elements, I doubt if many

of the newer designs do.<p>

BTW, Nikon won't say what ED glass is (except that it isn't fluorite).

Optically it must be similar because both Canon and Nikon produce high

quality super-tele lenses.

[louis_philippe_masse]

First of all, florite optical glass is not crystaline, it is

amorphous. This make a big difference in optical properties. The same

can be said about quartz elements used for UV transparent lenses. The

usage of "quartz" for this lenses is a misomer since quartz is a

crystaline form of silicon dioxide. "Quartz" lens are really fused

silica lenses. Fused silica is an amorphous form of SiO2.

 

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Secondly, the dispersion of light is a property shared at various

degrees by all transparent materials. Dispersion occures when light

of different wavelenght (colours) is diffracted (bended) differently

by the interface of a transparent medium and air (or another gas). It

is easy to find tables with dispertion power (represented by the

dispersive indices a.k.a. the Abbi number) for nearly all glasses and

transparent mediums. There is literaly thousands of different glasses

classified in several categories. Of thoses categories are fluor

crowns glasses or phosphate crowns. These glasses have high Abbi

numbers, hence a low dispersion power.

 

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Tersio: Note that dispersion of light has nothing to do with the

transmission of light. Transmission is a measure of the quantity of

light that passes through an optical element. Diffraction is

responsible for chromatic aberrations (lateral or axial).

 

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The bottom line is: Don't let the marketing dept. of camera

compagnies impresses you. All of the different manifacturer have

there own optical glass formulas. Canon uses fluorite crown glass,

Nikon may use a phosphate crown glass. Tamron, Sigma or the others

use different types glass with similar results. The only different

between the so-called "UD" or "ED" glass and their "super" varieties

is a lower dispersive power. To make a good camera lens, there is

more than good quality glass. What make the difference between good

and bad lenses is usually the optical design.

 

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Form more information on optics, I recomend "Optics" by Eugene Hecht

which should be easy to find in a good university library.

 

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Hope this helps.

 

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L.P.

[bill_meyer1]

Louis-Phillipe Masse, thank you very much for your reply, this was

something of the information I was looking for, your knowledge is much

appreciated!

[bobatkins]

Louis-Phillipe is incorrect about fluorite elements. Canon use

crystaline calcium fluorite elements. It's a crystal, not a glass

and isn't amorphous. It's not a fluorite crown glass, indeed it's

not technically a "glass" at all. Various types of fluorite containing

silica based glasses (crown glasses) do exist, and many of them may be

useable as

"UD" or "FD" or "ED" elements, but they aren't what Canon means

by "fluorite" elements. Crystaline calcium fluorite has dispersion

properties which are much better than any silca based glass in

correcting secondary spectrum, so you

can do with one fluorite element what might need two or more elements

of "lesser" silica based glasses, and even then you might not get

quite

as good a correction.

[louis_philippe_masse]

I done a small research on the subject and I must admit that Bob is right. Here is a small description of crystalline CaF2 provided by a Japanese manufacturer of synthetic crystals, OHYO KOKEN KOGYO CO., LTD.:

 

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Produced naturally as a fluorite, it has appropriate refractive index variations for wavelengths and transmission to wavelengths from vacuum ultraviolet 1250 angstrom to infrared 12 micrometers.

It can be used as a prism, lens, or window within a wide region from ultraviolet to infrared. It is a chemically and physically stable crystal with excellent water resistant, chemical resistant, and heat resistant characteristics. Also, since it has unique optical dispersion (Abbe's number: 95), it can be used as an achromatic lens (Apocromat) combined with other optical materials. It can also be used as a laser crystal or radiation detection crystal by doping it with the appropriate rare earth elements.

Two kinds are manufactured: for visible, infrared use and for ultraviolet use (non-fluorescent).

Recently, our company has succeeded in mass producing Calcium Fluoride and as a result, it has become the cheapest infrared transmissible material.

________________________________

 

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For my defense, I must say that only my first paragraph is wrong (except for the quartz part)! As Bob said, low dispersion crown glass can be used for photographic lenses. But most of these glass have Abbi numbers of the order of 40-50, which is lower than the value of 95 for fluorite (which is not a glass as I first though).

 

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All my apologies for the error.

 

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Louis-Philippe

[ed_turco]

As an amateur telescope maker, where can I get a piece of this stuff?

An internet search has gotten me nowhere as to a manufacturer. Price

no object; I know the stuff costs loads of cash, but I gotta build my

own APO.