Old lens and radiation

[majicman]

Wasn't here that I read that some old lenses could give off small amounts of radiation? I have tried to find it again but haven't been able to.

[John Parry]

Hi Majicman

Lens manufacturers (notably Asahi) used to add small amounts of thorium to their glass. It considerably increased the refractive index. It isn't a health and safety issue unless you sleep with one cuddled up to your ear, but it tends to turn the glass yellow. It's actually the same process that produces smoky quartz (morion?). If the lens was uncoated, you could turn it clear again by putting the offending element in the oven (the process is reversed by heat), but unfortunately at that time the lens people were also competing with newer and better coatings.

OK for black & white though!

Regards - John

[doubs43]

majicman said:

Wasn't here that I read that some old lenses could give off small amounts of radiation? I have tried to find it again but haven't been able to.

The CZJ Pancolar is another lens in which one element will turn yellow. Most people wrap aluminum foil around the sides and cover the front element, leaving the rear element exposed. The lens is then placed on a sunny window sill or somewhere outside to expose the element to as much of the UV rays from the sun as possible. It may take several weeks of daily doses but the yellow will slowly disappear and the lens will become clear once again. Pentax Spotmatic owners are well aware of this method generally.

Walker

[majicman]

Thanks guys I new I read it some wear in this forum But I couldn't find it this is all verry interesting. I guess I don't have anything to worry about, But I went and checked my lenses any way. See LOL.

[Peter S.]

Majicman,

You are not supposed to find a lot of radioactivity outside the lens as there is a lot of alpha
emission in the decay of Thorium (232-Thorium -> alpha -> 228-Radium -> beta -> 228-Actinium
-> Beta -> 228-Actinium -> 228-Thorium -> alpha 224-Radium -> alpha -> 220-Radon -> alpha
-> 216-Polonium -> alpha -> 212-Lead -> beta -> 212-Bismuth -> alpha or beta ->
212-Polonium or 208-Thallium -> alpha or beta -> 208-Lead - stable). The decay of the initial
Thorium is very slow (>10e10 years) and all the others pretty fast (between seconds and a few
years). Thus a dynamic equillibrium will be reached after a few years.

The alpha particles would not escape the lens, and most of the electrons should be absorbed
by the metal (in those days when Thorium was used in optics lenses were made of metal!).

I concluded for myself, that curing the yellowish tint by UV-light would restore the lens'
functionality to 100% - the topic was subject of a discussion with flamewar character on a
Minolta yahoo group a few weeks ago. I think these few million lead atoms would not have
a noticealbe impact on the glass - and btw. these two MC W.Rokkor 2.5/28 I got are better
than the MD W.Rokkor 2/28 I have, too.

Best regards
Peter

[kiev4a]

Setting the lens in a window (with tinfoil underneath to prevent fires) supposedly will clear up the discoloring. Exposure to UV I suppose. Never tried it myself.
The Russian I-61 L/D is of that type, too.

[mickeyobe]

Majicman,

I have a Pentax Super-Takumar 50mm f1.4 lens with a very pronounced orange cast to the glass. It does not appear to adversely affect the colour rendition of any pictures.
True, I am losing my hair but that started long before I acquired this lens.
Here is an example taken with a Spotmatic SP;

The white shirt is still pure white and the blue sky has not been degraded. Indeed, all colours seem quite accurate.
It also works very well on my Pentax K100D digital.
So I don't worry about it.

Mickey

[casualcollector]

A friend of mine was telling me of a reporter that visited the nuke plant he works for. After a trip through the plant, his equipment was geiger countered on the way out and the plant wanted to confiscate one lens that read higher than others. After a great deal of persuasion, they finally bought the thorium glass story and let him keep the lens. I've owned and traded off a yellowed Pancolar. I've also heard that the Canon FD 35/2 with concave front element uses thorium glass. My version of that lens is the redesigned unit from '76-'76. It has a convex front element and no thorium glass.

Bill

[paulatukcamera]

Mickey,


What beautiful colours - in the UK we certainly don't get the Autumnal shades you do.

Where was this taken?

Paul

[kiev4a]

I've never heard of the color cast of the glass ever affecting the actual photo.

[mickeyobe]

Paul,

This was taken at Pomona Mills a municipal park about a 10 minute drive from my house. The sugar maples and the staghorn sumac give us the most spectacular reds but the colours range from those brilliant reds, the maroon of Norway maples and the lovely gold of aspens and birch, all contrasted against the deep greens and blue greens of pine and spruce. And there is also the glowing golden hues of the dry grains and grasses.
We are blessed with spectacular fall colours and this past autumn was exceptional. Autumn is my favourite season, not too hot and not too cold and magnificent scenery wherever one looks.
But we make up for it in February. Right now we are beginning our 3rd week of deep freeze, well below zero C, with a possible snow storm coming in 18 to 24 hours. Toronto has had it easy. All around us this past week has seen up to 3 meters of snow and still more coming.
Sometimes I get carried away.

Mickey