[mccririck]

I noticed this one: SMC/S-M-C/Super-/Auto-/Takumar 55mm F1.8 Reviews - M42 Screwmount Normal Primes - Pentax Lens Reviews & Lens Database

It says the early ones had a radioactive glass element that yellows over time. How do you identify ones that are not the radioactive version, though?

[MossyRocks]

The radio active lenses have a thorium oxide glass element in them. I've know someone has a list of the old Takumar lenses that are radioactive that they will likely post since that happens almost every time this subject comes up. I know there is this list that has a bunch of radioactive lenses listed but I'm not sure how complete it is for old Takumars.

However it isn't something I would be concerned with as thorium has a very long half life (about the current age of the universe) and is an alpha particle emitter. Alpha particles (helium nuculus until it finds some electrons) are stopped by things as thin as a sheet of paper, or your outer layer of dead skin cells. So while they are radioactive they aren't very radio active (long half life means very few emissions) and the emissions are easily stopped. Unless you are grinding them up and eating them there really isn't cause for concern.

Their biggest problem is the yellowing that happens but that is easily corrected if you just set them in the sun for a few hours a day for a few days. The UV light will clears it up nicely. It is usually the rear element so have that facing towards the sun as glass blocks a lot of UV. Also if you put them in a window don't put them in a window that has low-e glass at that also blocks most of the UV as well.

[tvdtvdtvd]

This may help:

You Tube

You Tube

[sergysergy]

2:15 - I can confirm that once I was packing a takumar for shipping and the box started burning

[tvdtvdtvd]

He did make an interesting comment that is counter to what I always believed. He says the thorium is in the coating.
I always thought it was part of the make up the glass lens. Anyone know?

[stevebrot]

It says the early ones had a radioactive glass element that yellows over time. How do you identify ones that are not the radioactive version, though?

The various lists are your best reference (Google is our friend). As for the 55/1.8, as far as I am aware, all Super, S-M-C, and SMC variations should be considered radioactive and may require clearing of the brownish tint for best performance. The same is considered true for other than the 8-element version of the 50/1.4 Takumar.*

I own three lenses known for their radioactivity, two copies of the ST 50/1.8 and an M42 Auto Rikenon 55/1.4. All three had yellowed elements before I cleared them. The Rikenon, along with similar Tomioka-made f/1.4 and f/1.2 primes from the late-1960s on into the 1970s is one of the hottest lenses tested. I keep mine in a lead-lined bag (just kidding).

In regards to clearing...The brownish yellow color is due to chemical alteration (change of electron energy state) of the glass itself due to radiation. It may be cleared by exposure to sunlight (takes a long time), intense UV light (fast), or (very strangely) a day or so of close exposure to the IKEA JANSJÖ desk lamp (LINK). I used the JANSJÖ with excellent results and give it a strong thumbs-up.


Steve

* The 8-element version was the very first version of the ST 50/1.4 and was made in fairly limited numbers before being replaced by 7-element variants for the rest of the product life. The 8-element lenses are never radioactive, are collectors items, and usually command a premium price. There are numerous ways to tell the two apart (rear element profile, weight, lens barrel markings, etc.). It is sort of a cult thing. The lens review on this site for the 8-element is helpful for sorting things out, it you are interested. Super-Takumar 50mm F1.4 (8-element variant) Reviews - M42 Screwmount Normal Primes - Pentax Lens Reviews & Lens Database

[stevebrot]

He did make an interesting comment that is counter to what I always believed. He says the thorium is in the coating.
I always thought it was part of the make up the glass lens. Anyone know?

The coating story has had long legs, but no basis in truth. The composition of the glass is know and thorium oxide is in the mix in significant amounts. I suspect that people mistake the characteristic yellowish reflection of that era's lens coatings with radiation-induced discoloration seen when looking through the lens or the elements from a disassembled lens.


Steve

[stevebrot]

It says the early ones had a radioactive glass element that yellows over time.

Sorry about not addressing the actual text of the review. Yes, there is at least one report of a Super-Takumar 55/1.8 being non-radioactive (serial #885276 LINK). One of my lenses is close to the same serial number (#832885) and is classed as Super-Takumar 55mm f/1.8 (early) with production starting about 1962. Those early versions are characterized by flat focus ring (not scalloped) knurling, fine-ribbed aperture ring with smallest aperture at the right (opposite later lenses), and a "dot" as the index mark.

My lens was yellowed and responded to clearing. I have considered it to be radioactive, but have never had it tested. Aside from the photographic implications (less light transmission, color shift, and reduced optical performance) due to the browning, I don't see any particular hazard or disadvantage to owning a thoriated lens.*


Steve

* I suppose there is some risk associated with trying to board an aircraft with one of these in the bag.

[tvdtvdtvd]

The coating story has had long legs, but no basis in truth. The composition of the glass is know and thorium oxide is in the mix in significant amounts. I suspect that people mistake the characteristic yellowish reflection of that era's lens coatings with radiation-induced discoloration seen when looking through the lens or the elements from a disassembled lens.


Steve

Thanks for the clarification. I suspect you're correct on the origin of the confusion, as the videographer was focused on the
yellow reflection coming from the front lens coating. Yet we know the thorium element, in the 50/1.4 Tak at least, is the
rear element. I've personally tested my 50/1.4 with a geiger counter and can confirm it's much hotter at the back of the
lens than the front.

[stevebrot]

Yet we know the thorium element, in the 50/1.4 Tak at least, is the
rear element.

All THREE of the rear-most elements!


Lifted from: Radioactive Pentax Takumar lenses.


Steve

[pschlute]

I noticed this one: SMC/S-M-C/Super-/Auto-/Takumar 55mm F1.8 Reviews - M42 Screwmount Normal Primes - Pentax Lens Reviews & Lens Database

It says the early ones had a radioactive glass element that yellows over time. How do you identify ones that are not the radioactive version, though?

I ate a radioactive lens once and have had no side effects.

[stevebrot]

I ate a radioactive lens once and have had no side effects.

No yellowing over time?


Steve

[kypfer]

If the glass in the lens was significantly radioactive would it not have fogged the film in the camera?

[pschlute]

No yellowing over time?


Steve

Yes, but I put that down to my consumption of vodka.

---------- Post added 04-12-19 at 08:41 PM ----------

If the glass in the lens was significantly radioactive would it not have fogged the film in the camera?

This is the point. The radioactive risk is pretty much the same as wearing one of those glow-in-the-dark watches which were very fashionable at the time.....ie zero

[stevebrot]

If the glass in the lens was significantly radioactive would it not have fogged the film in the camera?

That is a common question and one without a firm answer. Google is no help.

There is significant gamma contribution, but with both mirror and shutter in the way and without knowing potential for fogging from a particular emulsion, it is hard to say. It is enough to note that while the radiation was no secret, I don't remember reading any film-fog warnings in the photography magazines of the day. I suppose some enterprising photographer might see fit to give it a trial of a month or two with some Tri-X.

Addendum: There is at least one amateur study that measured gamma from front, side, and back at various distances for several of the "hottest" photographic lenses. Both glass and metal caused significant fall-off; distance followed the inverse square law. Unfortunately it, like so many things, has disappeared from the Web.


Steve