Rare earths are generally found as oxides. As I have heard, "Rare" refers to the fact that it is very hard to separate the metal from the oxygen.
Rare earths are very heavy atoms. Silicon and oxygen (the main components in glass) are rather light. I suspect that the rare earths increase the refractive index of the glass because of this. I should probably google this before saying something stupid...
glad to see you here again! :) On your place I shouldn't believe what LZOS site says - that's exactly the place where the right hand doesn't know what the left one does :) J9 should be still in production, though they made maybe a million of them already? Anyway, I don't think that they brew glass for their photo lenses now. Maybe in case of J9 and, say, J61 they just assemble whatever they have inherited from USSR past...
PS. One of my friends who worked at LZOS someday told me that most of the color filters made under the brand of LZOS were in fact made by Pentax - optical glass factory in Izum was unable to output consistent quality, so the glass was imported :) The only local filters, he say, were the yellow ones, and not always!
Quite a few of the older Canon rangefinder's use this glass too. No wonder, I've never had children....
Many so-called Lanthanum glasses are actually Lanthanum/Thorium glasses. Many of the manufacturers preferred to emphasize the Lanthanum content and downplay the Thorium content. Period literature makes it clear that Thorium was intentionally included for the desirable optical properties of this glass: a high index of refraction with a low dispersion. This helps reduce chromatic and spherical abberations.
The radioactive lenses that I have measured are much to radioactive to be explained by the extremely slight radioactivity of Lanthanum, or by contamination with Thorium. The amount of Thorium is too large to be contamination.
Most major lens manufacturers used Thorium glass: Kodak, Voightlander, Schneider, Pentax, Nikon, etc. The Apo-Lanthars definitely contain Thorium. I don't know whether Russian lens manufacturers used thorium glass. One possible sign is that the glass ages to a tea color. This can be cleared via exposure to UV light.
Thorium glass is no longer used -- optically similar glasses without thorium are available.
For more on thorium glass, see my Aero-Ektar webpage: http://home.earthlink.net/~michaelbr...aeroektar.html
I have one of the radioactive thorium-glass Super Takumar lenses. Aside from yellowing (cause by decay products of the thorium, and I'm told that a few weeks under UV light will correct it -- trying to find an economical way to set that up without exposing the lens to the heat of direct sun), it's an excellent lens indeed - but based on my experience in the last few months, not enough better than my Tessars and Skopars to be worth the risks of grinding the thorium glass or the expense of properly disposing of the radioative and toxic residue.
As noted above, there are now glasses that effectively duplicate the optical properties of these radioactive glasses without the potential health issues. However, I'm not getting rid of my Super Takumar -- I don't store it in the headboard of my bed, or use it as a loupe, and I don't consider it any more hazardous, when stored with my camera gear and used on the camera as a lens is intended to be used, than ordinary photo chemicals used in their common applications.
Would I buy another one if it were crystal clear, clean aperture, and otherwise good condition? In a heartbeat, if I could afford one (and I might, before too much longer).
I remeber an article in the old Camera and Darkroom that discussed using a spcecial light to remove "impacted photons" from older lenses. I don't remember if it was specifically discussing lenses containing trace amounts of radioactive elements. I do believe that it was a UV light source. When I get time I will rumage through my stacks and find it.
Um, Jim, I have a small pile of lenses with elements containing Thorium. All had glass yellowed by radiation damage when I got them. The worst of the lot shot distinctly yellow EPP transparencies. Not good. A month or so of basking under a 20w fluorescent BLB tube cleared all of them, including the worst.
I rather like the "warm-tone" effect of my APO-Lanthar :)
It's weak, and only ovious when compared to the same scene shot with a "normal" lens. Combined with the smooth rendition the lens is (justifiably) famous for, it's a wonderful effect. I only wish I could afford some more of them - say a 210 and a 300mm...
Unfortunately, my Super Takumar 1.4/50 is yellow enough to give about 1/2 stop filter factor, accentuate clouds, and make color prints look muddy -- I've had to stop using it for color. Gotta get a UV light set up and see if I can clear it...
Donald, since you are in NC now, sunlight is strong enough this time of the year to leave it near a window and celar it in a couple of days.
Wrap it in aluminum foil to prevent overheating and use the most of UV