Yellowing Of Pentax 1970s Lens

Looking at some 67 lenses from Pentax and its common to see yellowing of the coatings from lenses made in the 1970s. Is it true that exposing the lens to sun or high concentration UV light helps get rid of the yellowing? I go to a medical high concentration UV-B light booth twice a week for tanning for psoriasis and wondering if I brought the lens along to expose it to the machines light, would help get rid of the yellow? Or should I just skip this lens as an option and look for one without? Only issue I have had with a yellow lens in the past was when I was shooting color, it gave a Kodak Gold yellow type tint to the pictures.

Skip it.

Several early and now novel 6x7 system lenses from the 1960s to 1970s had thorium90-tinted elements to improve contrast. So too did right-angle viewing magnifiers and central spot magnifier attachments. The internet is brimming with all sorts of cooked and whacky theories and quick fixes for yellowing, along with this stuff about your eyes glowing in the dark from radioactivity! Oh God.... Being a radioactive element, t90 cannot be removed from a lens, and exposure to intense light also does not eliminate it. If anything, these lenses serve as animated conversation starters! For a time I had an early Takumar with t90 and it did not give any yellow tint to Kodachrome slides in use at the time (mid-1990s).

braxus said:

Looking at some 67 lenses from Pentax and its common to see yellowing of the coatings from lenses made in the 1970s. Is it true that exposing the lens to sun or high concentration UV light helps get rid of the yellowing? I go to a medical high concentration UV-B light booth twice a week for tanning for psoriasis and wondering if I brought the lens along to expose it to the machines light, would help get rid of the yellow? Or should I just skip this lens as an option and look for one without? Only issue I have had with a yellow lens in the past was when I was shooting color, it gave a Kodak Gold yellow type tint to the pictures.

Click to expand...

Not necessarily, yellowing due to thorium elements happens only in the Super Takumar 6x7 105mm f/2.4. This coul dbe cured with UV or very intense light.

All other 6x7 lenses can have a yellow/orange sheen but nor radioactive yellowing.

So Scott, which lenses were you looking at had yellow tint to them? Was it the Super Takumar 105? I also heard that older Takumar lenses can have this yellowing, brought on by the thorium90 coating.

I never worked with Pentax 6x7 lenses but cured yellow tint from Asahi's S-M-C and SMC Takumar 1.4/50 (7 element) and Super Takumar 2/35.
6-7 weeks under inexpensive Ikea lamp fixed it. It also worked for a few Canon FL and FD lenses (1.4/50 and 2/35).

There's some confusion here. There's two types of yellow you could be seeing in a lens.

The first is a yellow tint of the anti-reflective lens coating. This is caused by the specific coating of the lens, and the composition and thickness of it. The coatings applied to camera lenses are completely transparent as far as the light passing through the lens are concerned, but because of the specific wavelength that the coating is optimized for, the light that bounces off the coating can appear to our eyes to be yellow, blue, purple or theoretically any color in the visible spectrum the coating is optimized for. This is all fine, and of no concern.

Rick Oleson has a good page on lens coatings on his site: https://web.archive.org/web/20180704141153/http://rick_oleson.tripod.com/index-166.html

There is then yellowing of specific glass elements in the lens caused by the presence of Thorium in the composition of the glass itself. This was done because the presence of Thorium allowed designers to make glass with a higher refractive index, in other words, you could make a thinner, less curved lens, that bends the light in the same way as a thicker more curved lens made out of a different type of glass.

The tradeoff here, is that because Thorium is mildly radioactive, the radiation will over a period of many years cause the glass to yellow. This may cause a yellow tint to be applied to the photos taken, but more importantly, it can also reduce light transmission through the lens. This person tested that, and found that it could be as much as a 50% loss, which is a full stop:
The yellowing itself is caused by electrons in the atoms of the glass being energized by the radiation, and jumping to a higher electron state. You can force these electrons to go back to their ground state and thus clear the lens, but to do so you need to actually energize them more so they can jump up and fall back down to their lowest state. By applying light you can accomplish this, and any light will work, but the photons of ultraviolet have greater energy than those of visible light, so use of UV light will do so more quickly.

As pointed out, it's easy to get rid of it -- and prevent it from reappearing. SEARCH for:

how to remove yellowing from thorium photographic lenses

and you'll find lots of suggestions -- most the same -- expose it to light.

1. Some, but not all, Pentax (and other) lenses contained thorium; your first step should be to determine which exact lenses, and how to identify them.

2. Yes, exposure to bright light, will reduce or eliminate yellowing of the glass elements IF due to thorium glass elements.

3. Depending on your light source, it may take days or weeks of exposure to clear the lens, so probably needs more time than you spend getting your light treatments.

As others have said, it may be yellowing / browning of the lens due to radiation damage from a thorium-containing element. See
https://www.orau.org/health-physics.../products-containing-thorium/camera-lens.html

This is not hazardous (as long as you keep the glass intact and don't try to grind it up or something), but it needs to be cleared. You don't have to remove the thorium from the glass, which would be impossible. Many people report that exposure to a bright LED lamp / something with modest blue or UV content works to reverse the damage. Sunlight should also work, though I wouldn't want to let the lens get really hot enough to heat up any grease in the focus helical.

I don't know exactly what the mechanism is here - it has to be some damage to the chemical bonds or amorphous-solid structure of the glass, but a damage that is itself reversible by light exposure. The radiation from thorium isn't strong enough to cause elemental changes in the atoms of the glass.

I finally got my hands on such a lens (a Takumar 50/1.4) but have not gotten to clearing it yet.