Yellowed lens

[anthonym-3]

I have a PENTAX 50mm f 1:4 SUPER TAKUMAR. Will the yellowing have the same or similar effect upon black & white film?

[titrisol]

Yes, it acts like a light-yellow or warming filter. However you lose 2/3 to 1 stop exposure.
You can cure it easily by exposing your lens to light either in a windowsill or with a bright white LED light for a short time.

I learned the trick in a Spotmatic discussion a while ago
Wim VanHeugen said "I use the alternative cure that was published on the Internet somewhere: expose the rear element to the light of a simple Ikea LED desk lamp. After a week or so the yelowing is gone.
Mostly it is stated that the lens element shouyld be exposed to UV-light but this desk lamp does not emit any UV!! Makes me think that just strong light does it. "

And here is a picture as well
http://forum.mflenses.com/fast-lens-de- ... 68896.html


And the explanation on how intense light helps curing the stain of radiation here: https://www.osti.gov/servlets/purl/10178461/
This report gives an overview of the effects of ionizing radiation on optical materials that may be used in spacecraft sensors. It introduces the relevant phenomena and indicates were more detailed information can be found. The topics covered include radiation induced absorption in ultraviolet transmitting materials, ordinary optical glasses, cerium stabilized optical glasses, and infrared transmitting materials; bleaching and annealing, and radioluminesence.

[alexvaras]

I have a UV nail dryer only for that, 3 days in a row and the yellowing it goes away

[Julio1fer]

This must work through the visible part of the light spectrum. Not much UV gets through glass. But it does work!

[GrahamS]

Exposure to bright sunlight over a period of three days will clear the yellowing completely. It is caused by the Thorium compound used in the glass mix, which is slightly radioactive. You can read more here: https://www.dpreview.com/forums/thread/4010268

[titrisol]

That is correct, intense light is the main clearing agent, as per the OSTI article I linked before.
This was tested in the 1940s and 50s, when it was observed that pyrex bottles exposed to radiation were browning and that the browning was cured after exposing to intense light either solar or artificial.

I was taught about UV exposure several years ago and yes 3-5 days in the windowsill worked fine for me; even in the Dutch winter 7 days did a lot of good. I learned about the IKEA lamp trick more rcently and tested it on my Super-Tak 50/1.4 and was pleasantly surprised as how quickly it worked (less than 24h)

This must work through the visible part of the light spectrum. Not much UV gets through glass. But it does work!