Removing the yellow tint in thoriated lenses? Anyone succeed?

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Maiku

Maiku
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Hi,

I have a 35mm f2 Pentax M42 lens that has yellowed from thethorium used in the coating process. The rear element is visibly yellow. I have read, on the internet, that I should set it in the sun for a few days and the yellow should disappear.

Has anyone had any success with doing this?

Does anyone have another suggestions to remove the yellow tinge?

Thanks,

Maiku
 
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It is the UV element of the sunlight that clears the yellowing. So a UV lamp does work quicker and also avoids any heat build up in the lens.

Having said that I have cleared a couple of 1.4 Taks which were yellow using the sun. Not easy in the UK in winter! I used a silver rear lens cap and left them pointing into the sun. Took a while but it works.

Kim
 
The shorter end of the visible spectrum and UV A seem to work best, while UV C has no effect. Curing/hardening/polymerisation lamps for nail studios and dentists and decorative black light bulbs work reasonably well, while UV-C lamps (sold for sterilizing applications, and in any case too dangerous, especially for photographers who should prefer just about any other crippling condition to blindness) will generally eat away plastic and rubber parts and bleach the lens barrel without removing any of the yellowing.

Bear in mind that the thoriated element often will not be the first or last, and that the light reaching it may be filtered by a preceding UV-blocking element, rendering the UV cure inefficient or slowing it considerably - it may be necessary to take apart the lens.
 
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Thanks guys for the positive feedback. I also have my eye on a Nikon Nikkor H 35mm f1.4 with the same problem. The Nikon would be nice to have. It could act as my standard on my D300. Again, many thanks.
 
Carter3John a RFF member has a photo on his flickr showing a rig he made used for clearing taks. I have used the sun but John's method is much better & takes far less time.
 
I bought a 51-UV LED light on eBay and it cleared my lens in under 48 hours. I bought a second yellowed lens and when I went to use the flashlight again I discovered the batteries corroded so much I had to throw it out. I bought another one on eBay and it didn't work at all. It was just too weak.
 
First. Dissamble the lens to get off the element/s that is/are yellowish. Generally is the last one.
Secind. You can put it under the direct sun or the UV lamp. Or the sun in the day and the UV at nigth.
Third. When the lens isn't yellowish, put it under UV light for a week each 3 months or so.
Forth. As you use the lens, less yellowish it becames.

My radioactive takumars are under UV light all the time. I take it off to use and when I be back at home, put it under the UV again. Work.
 
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