Canon LTM Lenses with thorium?

[brainmonster]

I know I'm going to get some pushback on this, but does anyone know if any of the Canon LTM lenses contain thorium? They seem to have an amber coating, but I'm assuming that's fine.

It seems like Canon's regular lenses like FD, etc., sometimes contained thorium for the faster lenses (Like 1.4 or faster).

Does this hold true for the Canon LTM lenses? I'm looking at purchasing a 50mm 1.8 and 135mm like in 3.5, I'm not sure how many versions/iterations these lenses have gone through, I know the older ones are labeled "serenar".

I think I would probably be safe with the lenses posted above, but does anyone know for sure?

Thanks!

 

[Peltigera]

Thorium emits alpha radiation. In air, alpha radiation can travel about an inch or so and cannot penetrate human skin so thorium lenses are quite safe to handle. Should you eat your lens then it is a very different matter as alpha radiation inside your body is rather dangerous.

 

[Paul Howell]

Here is link to a list of radioactive lens on Camerpeida. In general I would not hesitate to buy a lens with a thorium element, I have Pentax Super Takumar 50mm f/1.4 with thorium, sharp as a tack great contrast.

https://camerapedia.fandom.com/wiki/Radioactive_lenses

 

[flavio81]

I know I'm going to get some pushback on this, but does anyone know if any of the Canon LTM lenses contain thorium? They seem to have an amber coating, but I'm assuming that's fine.

Just a comment:

Usage of thorium has nothing, nothing to do with the coatings or the color of the coatings. This is a common misconception.

The thorium is in the glass itself.

As a rule of thumb, if a lens has a strong yellow or yellow-green color cast, it probably has one or more thorium elements. They can be de-yellowed by long exposure to light like an IKEA LED lamp (you can google it).

No reason to reject those lenses. Some of those are really good lenses.

 

[Paul Howell]

Your right, I should have wrote thorium element, my understanding is that thorium has a higher refractive index.

 

[flavio81]

Your right, I should have wrote thorium element, my understanding is that thorium has a higher refractive index.

It's not just higher refractive index.

When a designer chooses glasses, there are two main considerations:
- Refractive index (n): You want this to be HIGH
- Disperson (Vd): You want this to be LOW

(but not the only ones, since there are others, including more finer data about dispersion, cost, cost of machining, abrasion resistance, etc.)

Inexpensive glasses, that are suitable for use in camera lenses, have either:

- High n but high Vd
- Low Vd but low n as well.

The more expensive glasses are the ones that have high refractive index(n) AND low dispersion (Vd).

In the past one of those glasses were the glass types that contained thorium. Still, today a lot of specialist camera glass is made with a great variety of elements. There are some glasses that have almost no silica (si) in it!

So since thorium can be an occupational hazard for the glass workers, manufacturers, long ago, shifted to other glass types that can be as good or even better.

Most Inexpensive or reasonably-priced camera lenses are made with inexpensive glass in all or almost all elements. They can be very good as well and the lens designer has many ways to ensure high performance, not necessarily using expensive glass. The Nikon 50/1.8 AI long nose, for example, is a lens that has no expensive glasses (Source: patent research by Marco Cavina) yet it has an excellent reputation for high performance.

Expensive glasses can be really expensive and can drive costs up.

 

[AgX]

Thorium emits alpha radiation. In air, alpha radiation can travel about an inch or so..

It is more than an inch.

But yes, the danger is limited. A state institute on radioactivity once assumed the radiation strain by using an SLR with Takunar lens for 100h/year at 1/10 of the natural radiation.
But that also means, putting the plain lens under your pillow will give other results.

 

[Peltigera]

It is more than an inch.

But yes, the danger is limited. A state institute on radioactivity once assumed the radiation strain by using an SLR with Takunar lens for 100h/year at 1/10 of the natural radiation.
But that also means, putting the plain lens under your pillow will give other results.

But the lens is surrounded by a metal casing, the rear element has a metal camera between it and you. The thorium in the lens is encased in glass so the only radiation risk is from the surface of either front or rear element while you are handling the detached lens. It is not much aof a risk.

 

[AgX]

That assessment was based on metering with the lens mounted. But not all lenses are mounted. My Takumar for instance is stored open amongst other lenses, Though not under my pillow...

Moreover, one should not generalize concerning the direction of the radiation. I got for instance a german camera with a german lens, where it is the front end, not the back back end of the lens that radiates.