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09-22-2015, 03:49 PM   #1
Baronkatz
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Pentax Takumar Lens Radiation for Pentax 6x7 Lenses

Hello All!

I've been doing photography for 15+ years now. I shoot 35mm, Medium Format and Large Format. For most of my fashion and portrait work I use a Hasselblad 501C currently but am looking to get a Pentax 6x7, 67 or 67II with a 105mm F2.5 but I read on a site that the Takumer 105mm f2.5 is made with radioactive Thorium glass.

My question is whether this is for ALL the 105mm f2.5 lenses or only the older Takumar branded ones (the newer ones that just say Pentax are not radioactive)? When did Pentax switch over to non-thorium lens elements? How can I know which ones don't have it? Were there any other lenses that were radioactive?

Also I am debating between buying the three different Pentax camera's. The II is obviously more expensive but I want a really bright WHITE viewfinder. I can't stand it when they have the split finder and the yellowing. Do all of them have that? Thank you!

The reason for switching is that I really love the Bokeh, Contrast and look you get with the 105mm F2.5 Takumar and many other Pentax lenses, it's very similar to the 110MM/F2 Hasselblad Zeiss lens but for a lot less money. I can't afford the Hasselblad Zeiss lenses that I really want so this is a much cheaper option + it seems more ergonomic and compact to travel with.

09-22-2015, 05:13 PM   #2
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QuoteOriginally posted by Baronkatz Quote
Hello All!

I've been doing photography for 15+ years now...
My question is whether this is for ALL the 105mm f2.5 lenses or only the older Takumar branded ones (the newer ones that just say Pentax are not radioactive)?
Hi there. Are you questioning the safety of radioactive Pentax lenses? In all those 15 years have you known any reports of photographer's getting cancer and dying from these lenses? That would be a 105/2.4 lens, by the way.

If you purchase a Geiger–Müller counter and measure the output of the background radiation and that of the lens, you'll find it is safe to use when there are no solar flares going on. Otherwise, its probably best to keep it in a lead box until you're ready to use it. But if you are really, really concerned perhaps get the later version.

Last edited by tuco; 09-22-2015 at 07:15 PM.
09-22-2015, 06:03 PM   #3
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It was only the Super-Takumar 105 that used Thorium glass and they are not that common now. It was the only lens that Pentax used Thorium glass. The radiation level is very low BTW. Thorium was used because it had optical advantages over other glass types. The 67II has a bright finder so that may suit you.
09-22-2015, 11:12 PM   #4
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QuoteOriginally posted by desertscape Quote
It was only the Super-Takumar 105 that used Thorium glass and they are not that common now. It was the only lens that Pentax used Thorium glass.
The M42 Takumar 50mm f/1.4 also has a Thoriated lens element in it*. The Super-Takumar 105mm f/2.4 wasn't the only lens from Pentax that has developed a undeserved reputation for being dangerously radioactive**. If you kept one of these lenses in your pocket and wore those trousers 24/7 for a few years: then yes, your chances of developing cancer will be elevated but it wouldn't kill you....at least not directly***.

QuoteOriginally posted by First Poster Quote
When did Pentax switch over to non-thorium lens elements?
Circa 1968~ 1973 - when chemically pure Lanthanum salts were able to be manufactured on an industrial scale.In later years Lanthanum was eventually used as a substitute for Thorium. Eventually the process for refining the ore, Bastnäsite, which does contain considerable amounts of lanthanum (without thorium) helped improve processing to the point that it was no longer tainted by radioactive Thorium.


Radioactive element from an SMC takumar 50mm f/1.4 made circa 1966

* There are several actually, though there is a degree of debate over whether the addition of thorium in the lenses was intentional, as Lanthanum is easily tainted by Thorium as both metals are commonly found in the same ore, Monazite (which contains along with other rare earth metals, both Thorium and Lanthanum). With 1950s technology chemically pure Lanthanum was difficult to make on an industrial scale, such cross contamination was pretty much inevitable.

**Unlike some of the old Kodak Aero lenses, which are fully deserving of it.The pre-1950 Aero Ektar lenses are vicious alpha particle emitters due to heavy amounts of thorium used in the glass. Two elements found in the the two 7" and 12" f2.5 Aero-Ektars are made of glass with 11% and 13% Thorium by weight (bloody hell Kodak). While I don't have the study paper in front of me I recall that at most, 1960s era lenses from other 35mm lens manufacturers: Leica,Zeiss,Olympus,Pentax et.al only had 3~5% Thorium at most.

*** needless to say, I am not an expert on radiation. Take my advice with a brick of lead.


Last edited by Digitalis; 09-24-2015 at 06:39 AM.
09-23-2015, 02:13 AM   #5
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QuoteQuote:
Most smaller lenses with thorium elements are not very dangerous. However, thorium eyepieces are dangerous. They can give a very large alpha and beta particle dose to the cornea of the eye, causing cataracts and other problems. Normally these particles are stopped by skin, but the surface of the eye is quickly damaged by them, and at close range, the dose can be very high.
-- from Radioactive lenses - Camerapedia

Most likely, the alarmist sentiment and the stop in production comes from the cancer risk of those eyepieces.

As for yellowing, the web is full of people telling you to UV treat radioactive lenses to clear them.
09-23-2015, 10:46 AM   #6
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Digitalis-- Didn't know about the 50mm f/1.4 using Thorium. BTW, the Thorium mineral you mentioned, Mozanite is actually called Monazite.
09-23-2015, 07:42 PM   #7
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QuoteOriginally posted by desertscape Quote
Thorium mineral you mentioned, Mozanite is actually called Monazite.

Corrected, I should hire a proofreader. I was busy sifting through so many papers on Thorium, Lanthanum, and other radioactive substances that have found their way into photographic lenses.

Last edited by Digitalis; 09-24-2015 at 06:55 AM.
09-25-2015, 01:30 AM   #8
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I would like to buy maximum radioactive, yellowed 105/2.4. Anybody have it - let me know.
Just as collectible item.
I'm also going to sleep with taht lens below pillow.
I know physics quite well (I've been geophysician on Polar Expedition in Hornsund for one Year), so I know all this radioactive stories are just "mother tales".
Thorium glass indeed IS radioactive, but won't do my any harm - I'm sure.

09-25-2015, 05:48 AM   #9
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As for as de-yellowing a Super Tak 50/1.4 ... I use a small cheap Ikea LED lamp and shine on the lens for a week with a piece of tin foil between the lens and table top. There's noticeable improvement afterwards ... not perfect, but definitely improvement.

Wonder if that would be any good on the 67 lens ...
09-25-2015, 07:17 AM   #10
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I would like to have yellowed 105/2.4 lens.
That will be just fine (like use yellow filter) for B&W :P
09-25-2015, 09:04 PM   #11
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QuoteOriginally posted by MorgothV8 Quote
I would like to have yellowed 105/2.4 lens
that's a bit weak, go for a 1950's Aero Ektar - you could read from the glow of cherenkov radiation at night...
09-25-2015, 11:25 PM   #12
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Bo I won't be able to use it.
Seriously, I'll buy unfamous 105/2.4 yellowed sometime - thay're dirt chesp.
Just to have collectible/radioactive lens.
04-16-2016, 08:38 PM   #13
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QuoteOriginally posted by Baronkatz Quote
Hello All!

I've been doing photography for 15+ years now. I shoot 35mm, Medium Format and Large Format. For most of my fashion and portrait work I use a Hasselblad 501C currently but am looking to get a Pentax 6x7, 67 or 67II with a 105mm F2.5 but I read on a site that the Takumer 105mm f2.5 is made with radioactive Thorium glass.

My question is whether this is for ALL the 105mm f2.5 lenses or only the older Takumar branded ones (the newer ones that just say Pentax are not radioactive)? When did Pentax switch over to non-thorium lens elements? How can I know which ones don't have it? Were there any other lenses that were radioactive?

Also I am debating between buying the three different Pentax camera's. The II is obviously more expensive but I want a really bright WHITE viewfinder. I can't stand it when they have the split finder and the yellowing. Do all of them have that? Thank you!

The reason for switching is that I really love the Bokeh, Contrast and look you get with the 105mm F2.5 Takumar and many other Pentax lenses, it's very similar to the 110MM/F2 Hasselblad Zeiss lens but for a lot less money. I can't afford the Hasselblad Zeiss lenses that I really want so this is a much cheaper option + it seems more ergonomic and compact to travel with.

Thorium is not very radioactive in its natural state, compared to Uranium oxide. Gas Mantle globes were made of thorium and they were deemed safe for decades. Most radiation can only penetrate about 2 cm of air. I doubt that the gamma rays of the small amount of Thoria in the lens have enough energy to escape the glass. Over time the lens will yellow, from radiation however.

You probably get a lot more radiation from building bricks and granite counter tops. They will make a scintillometer scream. Counts of granite slabs can be 6,000 per minute, while background is about 40 counts.

"Thoria is a material for heat-resistant ceramics, as used in high-temperature laboratory crucibles. When added to glass, it helps increase refractive index and decrease dispersion. Such glass finds application in high-quality lenses for cameras and scientific instruments. The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal. Yellowed lenses may be restored to their original colorless state with lengthy exposure to intense ultraviolet radiation."

Just don't sleep with the lens in your underwear. You should be fine.

Last edited by frodobaggins; 04-16-2016 at 08:51 PM.
04-16-2016, 10:52 PM   #14
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QuoteOriginally posted by frodobaggins Quote
Thorium is not very radioactive in its natural state
That is because it is never found in its pure elemental form. It is always mixed in with other minerals, radioactive decay products and isotopes.

QuoteOriginally posted by frodobaggins Quote
Most radiation can only penetrate about 2 cm of air.
Alpha particles: yes, they can be stopped by air alone. Beta particles, you will need something denser than air to stop those. Gamma particles, you need Lead bricks to effectively shield yourself from them.

QuoteOriginally posted by frodobaggins Quote
They will make a scintillometer scream. Counts of granite slabs can be 6,000 per minute, while background is about 40 counts.
Count rate does not universally equate to dose rate, and there is no simple universal conversion factor. Any conversions are instrument-specific. So your statement is basically useless, Luminox watches can set off scintillometers too you know. And people have those strapped to their wrists for years.

Last edited by Digitalis; 04-18-2016 at 06:39 PM.
04-17-2016, 10:13 AM   #15
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There are youtube video's where it's more or less proven that the camera body will stop most of the radiation. Even just putting caps on the lens will help.

This has literally been discussed to death, Unless you are sleeping on the lens you're going to be fine.
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