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.

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?

-- Bill Meyer (william_meyer@stortek.com), September 29, 1998

Answers

I'm to lazy to copy Canon's text, it several pages long, but if you get ahold of a Canon EF lenses L series catalog at a good pro dealer, it has a prety good explanation.

-- Bill (Bill.Wyman@utas.edu.au), September 29, 1998.

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.

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.

-- Brad (reloader@webtv.net), September 29, 1998.

Instead of fluorite, Nikon uses their ED (Extra-low Dispersion) glass. Nikon prefers ED glass to fluorite because:
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.
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.

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.

-- Geoffrey S. Kane (grendel@pgh.nauticom.net), October 01, 1998.

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.

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.

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).

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.

Form more information on optics, I recomend "Optics" by Eugene Hecht which should be easy to find in a good university library.

Hope this helps.

L.P.

-- Louis-Philippe Masse (phisa@generation.net), October 07, 1998.

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!

-- Bill Meyer (william_meyer@stortek.com), October 14, 1998.

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.

-- Bob Atkins (bobatkins@hotmail.com), October 20, 1998.

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.:

________________________________ 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. ________________________________

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).

All my apologies for the error.

Louis-Philippe

-- Louis-Philippe Masse (phisa@generation.net), October 23, 1998.

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.

-- ed_turco (ed_turco@yahoo.com), January 01, 2002.