I recently received the following question:

Q: “Do you know if the lens Pentacon 50mm 1.8 is a radioactive lens?  Does it have thorium?”

Here is my answer:

A: This lens was manufactured in an M42 mount for Praktica 35mm cameras and in the Praktica bayonet mount for the subsequent Praktica B-series cameras.  It is not a lens for medium format cameras such as the Pentacon Six.

Many lenses made from the 1940s to the 1970 contain thorium, although this is normally in extremely small quantities and presents no health hazard. There seems to be a pretty comprehensive list of lenses that contain thorium here:
http://camerapedia.wikia.com/wiki/Radioactive_lenses.  Website accessed on 2nd May 2016.  I have no connection with the writer of the page in question and do not have the scientific expertise necessary to evaluate it.  I therefore take it at face value.

According to that article, the main cause for concern is the presence of thorium in viewfinder eyepieces, which are of course held extremely close to the eye. I have, however, seen no reports of thorium in the eyepiece of any Pentacon or Praktica camera.

The most well-known Carl Zeiss lens containing thorium is the Carl Zeiss Jena 55mm f/1.4 two-pin Pancolar lens that was made principally for the Pentacon Super, a high-specification and extremely expensive 35mm camera that was only on the market for approximately four years, between 1968 and 1972.  The presence of the thorium in this lens can be spotted by the naked eye in that some yellowing of the lens elements has occurred.  These cameras and this lens can still be found, but they usually fetch very high prices and they are generally only of interest to collectors.

To return to the lens about which you ask:
In the 1970s and most of the 1980s, the name “Pentacon”, as well as appearing on cameras, was only given to non-Zeiss lenses, which essentially meant lenses from Meyer-Optik in Görlitz, East Germany. There is no evidence that any of the lenses from that manufacturer contained thorium and no lenses branded with the name “Pentacon” are in the list on the above link, although the more expensive Carl Zeiss Jena 50mm f/1.8 Pancolar, “Zebra” version, only, i.e., mid 1970s, is on the list.  That, too, is a lens in M42 mount for Praktica 35mm cameras.

The above web page does list two Carl Zeiss Pentacon Six lenses that contain thorium, the 50mm Flektogon and the 80mm Biometar, both in the “zebra” version only.  I do have a Biometar in the “zebra” version and have not observed any yellowing of the lens elements.  I do not know whether or not this is a reliable indication of the amount of thorium in the lens.  You will notice that the above web page also lists lenses from most of the major camera and lens manufacturers, including Canon, Leica, Minolta, Nikon, Pentax, Kodak and many others.

It is reported by the above web page and other websites that it is possible to reduce the yellowing of lens elements by placing the lens in the ultra-violet light of the sun for a period of several days.  However, I would make two comments on this:

• beware of overheating the lens, which could cause lubricants to “migrate” from the focussing helicoid onto the aperture blades, causing them to stick;
• glass filters ultra-violet rays.  I therefore wonder if the window should be open, to avoid reducing the effect of the UV rays, in which case security needs to be considered – you won’t want a stray cat to knock your lens onto the floor!

I am not a scientist, but understand that all items emit radiation, although levels are in most cases so low as to be insignificant. The above website states that when the lens is 3 feet (0.9 metres) away, the level of radiation cannot be detected because it is no higher than the general background levels in the environment.

Obviously, when a lens is on a camera that is being used, or even in a camera case at one’s side, it is closer than 0.9 m to the body.  However, I understand that for the short periods of time that this is the case – for instance, perhaps a few minutes repeatedly over the course of a few hours on a single day or even the same amount of use over a period of several months when one may go out with the camera – this has no measurable effect on the user.  In many countries of eastern Europe and in many other parts of the world, the Pentacon Six was used by professional photographers, presumably on a daily basis, for many years or even decades.  Again, no reports have been observed of any concerns about the constant use by these people of lenses with thorium in them.  Nor am I aware of any concerns ever having been expressed with regard to professional photographers who daily used other major photographic brands that also employed thorium as a component in some of their lenses.

If you are worried about this, I suggest that you dispose of any lens that you discover contains thorium.  However, this does not, in any case, appear to include the 50mm Pentacon lens that you mention.

With best wishes

“Mr Pentacon Six”

May 2016

Safe disposal of radioactive lenses: suggested sources of information

In late December 2016 I received the following question on this topic:

Re: Smc takumar 35mm f2.0
Q: “I have purchased the above mentioned lens last week, but just found out it has thorium in the glass. I'm concerned and wish to dispose of the lens safely. Can you please advise?”

Here is my answer:

28 December 2016