PSA: If you have a yellowed lens that you'd like to use, great news: you can fix it: http://pentax-manuals.com/repairs/yellow.htm http://pentax-manuals.com/rep

PSA: If you have a yellowed lens that you'd like to use, great news: you can fix it: http://pentax-manuals.com/repairs/yellow.htm http://pentax-manuals.com/repairs/yellow.htm http://fockert.xs4all.nl/~pentax/radioactive_lenses/radioactive.html http://fockert.xs4all.nl/~pentax/radioactive_lenses/radioact... This pops up with some regularity in the Pentax world. Turns out Pentax made a lot of radioactive lenses back in the day[0], and they sold them in considerable quantity. I happen to own one (a Super-Multi-Coated Takumar 50mm f/1.4), and can confirm that sunlight works to clear the yellowing. If you're trying to avoid radioactive lenses, I believe pretty much everything Pentax made after the SMC PENTAX-M is non-radioactive. I have a SMC PENTAX-M 50mm f/1.4 as well that shows no signs of yellowing. [0] https://news.ycombinator.com/item?id=20612636 https://news.ycombinator.com/item?id=20612636 <- scroll down ginko's link, there are scads of them.