Pentax lenses, Thorium problem?

[LeicaFoReVer]

I found a pentax spotmatic in an antique shop for 30$ including takumar 55mm f1.8 lens. It performs very nicely and it works. But I realized that the shots I made have a yellowish tint on about what I heard on the net that some old pentax lenses turns yellow due to thorium on the coatings.

The lenses mentioned were 35mm and 50mm f1.4. I did not hear 55mm 1.8.

Is there anybody who experienced that as well?

Here is the photo. Note that how skin tones are highlighted. I removed the tint on picasa.

 

[ZeissFan]

Supposedly, if you leave the lens out in the sun it will reverse the yellowing. How can we see the extent of the yellow cast if you remove it?

 

[Sonnar Brian]

I suspect that a UV light will also bleach out the radiation damage to the glass. I left my 50/1.4 under a UV lamp. Does not generate as much heat as sunlight, and the glass absorbs the deeper UV of the lamp. The thorium Summicron I bought had been sun-bleached, and the helical was very stiff. The grease had been cooked. I relubed it.

With that said: Canada Balsam can also yellow, and cause similar color shift. Check the glass with a light.

 

[charjohncarter]

I'm not sure you will get a yellow cast with the radioactive lenses unless you are using transparency film. And ZeissFan is right leave it in the sun for a long time or try this:

"How to Cure Yellowing in Takumar 50mm f1.4 lenses. Remove the lens cap and wrap the uncapped lens, except the rear element, in aluminum foil and then set it on a windowsill that faces towards the sun. Prop it up with something (like a rolled-up towel) so that it tilts in order to get the most sunlight into it. Then go away and leave it. A mild case takes about a week to ten days on a windowsill in California. A severe one takes about a month. The foil serves two functions. It reflects light off the lens body and thus reduces heat buildup that otherwise would occur due to the black finish on the lens and this could affect the lubrication inside the lens. And by not capping the lens before wrapping it, light makes its way from the back of the lens and reflects back off the foil at the front of the lens, thus attacking the yellowing again."

I also hear that doing the above but using a coiled fluorescent bulb will work. Window glass cut UV so it will take longer.

 

[Sonnar Brian]

>I'm not sure you will get a yellow cast with the radioactive lenses unless you are using
>transparency film.

You can! I can see it on C41 processed color film.

 

[charjohncarter]

I stand corrected.

 

[f16sunshine]

I've seen it on DSLR as well. Although can be fixed with WB adjustment. The method John Carter describes has worked for me. Even with one very Extremely yellow Super Tak 1.4/50 that took a couple of summer months. I tight wrap of clear plastic wrap around the whole lens/foil shabang will keep dust out but not those productive UV rays. A month or two on a window sill can be pretty dusty for an open rear element. Make sure the plastic does not make contact with the glass. Best to turn the lens to minimum focus before foil and plastic wrap.

 

[squirrel$$$bandit]

Some of the 50/1.4's have this problem, and UV or direct sunlight (like, a month of direct sunlight) will cure it. The thorium does indeed yellow one of the elements.

But I'm not sure that this ever happened with the 55/1.8. I don't think so. The yellow cast may be the result of the processing you're using, and any yellowish look to the glass may be the coating.

 

[charjohncarter]

Some of the 50/1.4's have this problem, and UV or direct sunlight (like, a month of direct sunlight) will cure it. The thorium does indeed yellow one of the elements.

But I'm not sure that this ever happened with the 55/1.8. I don't think so. The yellow cast may be the result of the processing you're using, and any yellowish look to the glass may be the coating.

This is what I was thinking. What I meant was scanning could have some algorithms for auto leveling and color correction, maybe they don't, but you would not see these on board changes when looking at a slide.

 

[rover]

But I'm not sure that this ever happened with the 55/1.8. I don't think so. The yellow cast may be the result of the processing you're using, and any yellowish look to the glass may be the coating.

Ditto, I am pretty sure the 55/1.8 isn't a radioactive lens, just those early 1.4s.

 

[Doug]

It's questionable whether the 55/1.8 had a thorium-glass element, but it may be so... especially if you're holding the evidence in your hand! 🙂 This website:

http://www.orau.org/ptp/collection/consumer products/cameralens.htm

...has some useful information including a list of lenses known to include thorium glass, leaving open the possibility of yet-unknown others. Some from Pentax: SMC Takumar 50mm f1.4, Super Takumar 35mm f2.0, 50mm f1.5, 55mm f2, so perhaps the f1.8 as well. However, there may be a misprint in the list, since AFAIK there was no f1.5 lens from Pentax.

 

[LeicaFoReVer]

well I am sure it is not from scanning. I check through the white wall. Besides it is obvious that the yellows are highlighted like I used yellow filter.

Is there any health concerns related to thorium content? Otherwise it is not so crucial for me. It is like a permanent yellow filter I dont need to buy 🙂

 

[charjohncarter]

Doug, I also found this:

There several versions, but without optical difference, which is good because
the 7-element 50/1.4 Takumar design is a legitimate claimant to the title of sharpest
normal lens ever made; and bad for another reason, noted below.
There was a cosmetic change from scalloped knurled focus grip, to the
rubberized one which was cheaper to make. This was simultaneous with
the name change. Some users have also noted that the "feel" of the
focusing on the earlier, Super-Multi-Coated version is silkier and
smoother, both when new and as the lenses age. I can't remember a
reason why this should be so, but I haven't taken one of the SMC version
apart recently, either.
The bad news: unfortunately, both versions, and the SMC Pentax 50/1.4
with 52mm filter ring, have an internal element made of material known as "hot glass"
which turns yellowish to brownish with age. The affected lens element contains
thorium, a radioactive rare earth element. Several lens makers including
Pentax, Kodak, Leica, and Canon used this manufacturing technique in the
1940s to early 1960s to lower the index of refraction of specific lens elements.
The yellowing is curable by exposure to ultraviolet or sunlight, as has been
discussed on the Spotmatic Yahoo! Group.The earlier 8-element 50/1.4
Super-Takumar and the various 55/1.8, 55/2.0 and 55/1.9 Takumars do
not turn yellow, nor do the newer 50/1.4 SMC Pentax-M/A/F/FA.

 

[LeicaFoReVer]

Well, I hope no, I hope I see wrongly. I will try to post an image of the lens under white light. Maybe it was CLA'd in the past and some elements had been changed. Who knows...

 

[charjohncarter]

Well, I hope no, I hope I see wrongly. I will try to post an image of the lens under white light. Maybe it was CLA'd in the past and some elements had been changed. Who knows...

My problem is I have a radioactive lens (40 years), and it is yellowed. And the last thing that I have a problem with is a yellow cast (actually no other problems). All I ever noticed was just a warming of the transparencies that I have taken with it. I actually liked that (this is in tungsten light, too).

 

[charjohncarter]

I'm not sure I wanted to do this but I cured my Thorium 'yellowing.' Here is the cure 15-20 hours:

 

[bmattock]

My Super Takumar 55mm f/1.8 has yellowing, the Super-Multi-Coated 50mm f/1.4 does not, in direct opposition to what is said about them. No idea why.

 

[tedwhite]

I have posted some time ago on the Yahoo Spotmatic Users Group a step-by-step set of instructions for de-yellowing the 50/1.4 using the aluminum foil method. Here in Arizona (lots of sunlight) it took about 10 days. These are very good lenses and worth the time and effort (it's actually really quite simple) to bring them back to original shape. A number of years ago I had one that was very yellow; however, I only used it with black and white film so it made no difference (as far as I could tell).

 

[jan normandale]

Hi Ted , HNY!

 

[radiocemetery]

Pretty spiffy set up John. What temp (K) is the compact flourescent that you used? I have a 50 1.4 that would benefit from this treatment, it is noticeably yellowish brown when looking through the lens against a white background. BTW I have two 55 1.8's that do not show this yellowing. They are Auto-Takumars and the 50 1.4 is a Super Takumar. Happy New Year.

Steve