If you’ve ever noticed that some mid-century lenses like certain Canons, Nikons, or Pentax models, have developed a warm yellow or amber tint over time, you’re not imagining it. It’s not dirt, haze, or coating degradation. It’s physics.
Matt Duclos breaks down the phenomenon beautifully in his new article on The Cine Lens:
“Thorium-Induced Yellowing in Mid-Century Lenses – Cause, Effect, and Solutions”
Read it here: https://thecinelens.com/2025/09/18/thorium-induced-yellowing-in-mid-cent...
Here’s the short version:
From the 1950s to the 1970s, manufacturers used thorium-doped glass to improve refractive index and chromatic correction. Over time, the radioactive decay of thorium creates microscopic defects in the glass called color centers (or F-centers). These defects absorb blue and UV light, which is why the glass takes on a deep yellow-brown tint and loses transmission.
Fortunately, this can be reversed through photo-bleaching, using ultraviolet light to excite and re-stabilize the trapped electrons, restoring the glass’s original clarity. The process is slow but safe, and when done correctly, can recover the lens’s true color balance and light transmission without damaging coatings or components.
Beyond being a fascinating bit of materials science, this explains why certain vintage lenses—especially those prized for their “warm” look, may actually be unintentionally color-shifting due to decades of atomic-level change.
Have you encountered or restored a yellowed vintage lens before? Did you leave the tint for its character, or try to bring it back to neutral? How do you feel about preserving vs. correcting the visual quirks of aging glass?
2 people like this
This is really interesting and I've actually head about both, but have never seen the UV light bleaching of the radioactive lenses. But I have seen fungus infested lenses. I've even fixed some of my lenses that had not yellowish tint but simple haze oz. were there was kind of white covering on the lenses. The problem is that when I cleaned them sometimes the filter coating would get wiped off.
In either case manual vintage lenses are much nicer to fix and repair than newer lenses with electronics, which are way too complicated anyway.
2 people like this
This is fascinating!
3 people like this
Had no idea this was why - I saw this happen in my father's old camera lenses and never thought anything of it. Very cool Ashley Renee Smith
2 people like this
What!? This is freaking awful. You mean I've been irradiated by my dad's super-8? No wonder I'm so weird nowadays.