Removing Yellowing?

[Michael Panfeld]

Hi: I've come across several posts on still camera forums that talk about removing the "yellowing" from older lenses by exposing the elements to UV light (usually by leaving them out in the sun). Most of them also discuss that the yellowing is due to the use of radioactive elements (such as thorium or other rare earth radioisotopes) during the 40s through the 70s.

Can anybody shed light (no pun intended) on both the cause and the alleged cure for "yellowed" lens elements? Has anybody tried this on a cine lens with positive results?

Thanks

 

[Stephen Williams]

Radioactive decay, a geiger counter will confirm the lens is slightly or in some cases very much more radioactive.

 

[Matthew Duclos]

The whole thorium thing is in fact true. Not all glass was made this way.
But it will definitely contribute to yellowing.
As for the whole UV thing...
I have always heard that technique being used to "rid lenses of fungus".
UV light may slow or stop the growth of fungus, but it will not remove it.
The only way to remove fungus is to physically polish it of the element itself.
That being said, UV light will not remove yellowing.

 

[Davide B.]

This happens with the relatively popular Pentax Super Takumar 50mm f/1.4 lens. Here are some links to how users deal with it.

http://photo.net/pentax-camera-forum/00RYOO

http://orlygoingthirty.blogspot.com/2008/07/radioactive-thorium-glass.html

I have a 50mm f/1.4 Super Takumar that I use a lot. It has yellowing which I never treated because I find it just as easy to white balance the problem away. One thing that does give me pause is the point brought up in the orlygoingthirty link I gave you which notes that the yellowing seems to soak up light, turning a 1.4 lens into a 1.8.

davide

 

[Michael Panfeld]

OK, so that's great news. Time to try it on my still/cinema lenses (i'll try it on my Canon FD 35mm chrome-nosed thorium lens first). Question:

If the UV light cures yellowing over time and also kills the dreaded fungus, why shouldn't I simply store my lenses in a UV light bath when not in use or at least once in a while to maintain them? Is there any long-term effect on the glass elements or the lens mechanics/body to long-term UV exposure? If not, heck I might even add a UV light to my Pelican case (thinking LED if they make one due to battery powered and no heat).

Thanks

 

[Steven Steinhardt]

I recently purchased a Pentax Spotmatic II and a 50mm f1.4 Super Takumar lens for wedding photography here is Los Angeles. The lens was purchased off Ebay for $60. It was in great condition but had bad yellowing as well. I wrapped it in tin foil and left it on the windowsill for about 3 days, checked on it and nothing changed. The window just filters out too much UV. So I went to Home Depot and bought this plug in GE Black light. It is 22"" T8 form fixture. I put the lens in a cardboard box lined with reflective aluminum foil. Placed the foil wrapped lens with the rear element facing the tube (the tube was about 4-5" away. After just 24hrs, the lens looks crystal clear. the viewfinder image now looks much brighter, and I see no color cast between what my eye sees and what I see through the lens. Blacklights are the way to go with these things.



-Steve Steinhardt

Blog

 

[Dan Hudgins]

UV?

UV?

I know some kinds of plastic and resins yellow FROM exposure to UV light.

The radiation from the glass could act like UV light on the lens cement.

What is the chemical mechanism that people think UV light is using and is it acting on the glass, coating, cement, or smoke that has collected particles on the surface of the glass.

Maybe UV can break up cigarette smoke deposits as it can dislodge atoms, the dislodge of atoms is how it makes plastic yellow.

Radiation can make glass yellow by dislodge of atoms, that was what happened to Pioneer 10 when it flew past Jupiter, its camera lens became almost black. I wonder if airport X-ray or neutron scanners can reduce the T/ stop also in some glass types?

Anyway, there may be more than one reason that lenses are yellowish, high index glass can be almost orange color, Kodak had big problems with that in the 1940's. So there may not be a single way to reverse yellowing, and UV may make it worse depending on the type of cement and the amount of UV you expose the lens to.

Pulsed IR lasers may be able to remove dust and other deposits on the lens through conversion to C02 etc., but might over heat the cement. Taking the lens apart to clean it can mess it up since the elements will never be exactly in the same place as they were.

Glass in general does not pass short wave UV, so almost no short wave UV could make it past the first element, that is why quartz lenses are used for shooting UV photos.

Putting a lens in the sun, might heat it, and heat can soften the lens cement and dry fungus. You might get the same effect keeping the lens in a warm dark dry place. The UV tube was also warm if the lens was close to it, some IR may pass through the UV dye filter on the tube as well?