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06-07-2008, 02:19 PM   #1
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Why does sunlight clear yellowed lenses?

Okay, here's one for the scientists among us:

It's well known that some of the older Takumars will have elements take on a yellow tinge if stored in the dark for a long time. Lore (meaning I haven't seen an official citation on the web) indicates that this is because of radioactive substances in the elements that help with light dispersal.

Okay, so the fix is to set the lenses out in strong sunlight for a couple of weeks, perhaps wrapping the lens body in foil to keep it from overheating.

I've just finished doing this to a couple of really nice SMC Taks (early) and sure enough, the yellow cast has faded to nearly nothing. Glass is like new.

So... just out of curiosity, why is it that a couple of weeks of sunlight has this effect on solid glass? UV?? Heat??

Thanks,

germar

06-07-2008, 03:15 PM   #2
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As I understand it, the UV reverses the tingeing which is caused by the bombardment from alpha particles within the glass changing its composition slightly. A sunlamp would clear it more quickly.

Heat is to be avoided. It may degrade the grease in the lens causing the aperture leaves to get stuck.
06-08-2008, 12:24 AM   #3
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If you are using a DSLR, you don't have to care about the yellow tint. Whitebalance will take care of it.
06-08-2008, 05:05 AM   #4
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Because of how the Pentax' AWB works, I would certainly remove that yellow tinting. At least for indoor shots.

06-08-2008, 06:36 AM   #5
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QuoteOriginally posted by KjetilH Quote
Because of how the Pentax' AWB works, I would certainly remove that yellow tinting. At least for indoor shots.
Or use rawshooting and custom wb
06-08-2008, 07:32 AM   #6
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QuoteOriginally posted by Zewrak Quote
If you are using a DSLR, you don't have to care about the yellow tint. Whitebalance will take care of it.
That really depends upon what you want, Yellowing will cause some lenses to give you an OLD PHOTO appearance.I did Not have to edit this photo for this appearance.

Taken with a 35mm Super Takumar f2.0 lens

I forget what f-stop I used, but the rest of the data is there.
06-08-2008, 10:59 AM   #7
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QuoteOriginally posted by little laker Quote
That really depends upon what you want, Yellowing will cause some lenses to give you an OLD PHOTO appearance.I did Not have to edit this photo for this appearance.

Taken with a 35mm Super Takumar f2.0 lens

I forget what f-stop I used, but the rest of the data is there.
See, everything can be used to an advantage. . Great shot.

But are all clouds in Canada that ugly? Whats up with the odd line at the top of the picture. The cloud looks misconfigured.
06-08-2008, 11:24 AM   #8
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Those are thin cumulus, so looking at an angle, they look like they have flat top. Actually, the base of the clouds is always flat. The top is generally "fluffy", but when they are thin, they look flat.

06-09-2008, 06:51 AM   #9
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I recently bought a Super Tak 50/1.4 from KEH (EX grade). It has some yellowing, but I have yet to develop any of the photos taken with it (I use film only). Do all yellowed lenses produce a yellow tint in photographs? I wonder if KEH tested it to see if there were any tinting of images. All they would have to do is mount it on a DSLR, and snap a few shots.

Thanks,
Glen
06-09-2008, 06:59 AM   #10
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Depending on the amount of tinting you may not want to remove it. I have a SMC 50 f1.4 that produces the most wonderfully warm tones that I'm careful not to ruin the effect. It sees just enough light to maintain it's tone.
06-09-2008, 07:13 AM   #11
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Thanks, Tom. I will take the lens out of the sun now.

Glen
06-09-2008, 10:07 AM   #12
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QuoteOriginally posted by Zewrak Quote
See, everything can be used to an advantage. . Great shot.

But are all clouds in Canada that ugly? Whats up with the odd line at the top of the picture. The cloud looks misconfigured.
I think that flyer did a great job at answering your cloud question. Thanks flyer

I actually got tired of the effect, and did the sun treatment. I still get the OLD photo appearance, but I don't get the heavy grain.

I didn't really mind the grain, except there was just a little too much for my taste
06-09-2008, 12:42 PM   #13
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QuoteOriginally posted by dosdan Quote
As I understand it, the UV reverses the tingeing which is caused by the bombardment from alpha particles within the glass changing its composition slightly. A sunlamp would clear it more quickly.

Heat is to be avoided. It may degrade the grease in the lens causing the aperture leaves to get stuck.
Nice photos! But maybe the issue isn't whether adding a "yellow filter" to the lens is desirable (the photos clearly demonstrate it could be!)...but rather that a good photographic tool/lens minimizes any unadjustable biases. If a lens has yellowed, it is therefore less desirable than an identical non-yellowed version, plus a yellow filter on top. After all, it is easy to add and remove the filter as desired!

Also, had a little to add about the original question .

With all due respect, dosdan, I believe what you advised above is wrong. Alpha particles are a form of highly ionizing radiation, and as such, interact quickly with the environment and are dissipated...this short lifetime ensures that sunlight has no significant alpha radiation (it is exceedingly unlikely that reactive particles would make it across the solar system and through Earth's atmosphere). Rather, light itself (ie, photons) can transmit energy, which can be captured by a chromophore. Higher frequency light (ie, UV) carries more energy than lower frequency light. Sunlight carries energy distributed across a spectrum of frequencies, including UV.

UV light, as in sunlight, can be absorbed by a chromophore, which must then dissipate the energy. Often, this is done by reacting away part of the chromophore. In the case of a yellowed lens, the yellow part is probably some organic deposit, whether from grease, dirt, old coatings, etc. Obviously, it absorbs some amount of light in the visible frequency, because we can see it as yellow. If you can remove the yellowing by exposing it to UV light (as in sunlight), then it also absorbed in the UV frequencies. So why does visible light not react away the yellowing, over time? Because UV light is higher energy, and there is probably some threshold at which the UV light can react away the chromophore, but the visible light cannot.
06-09-2008, 06:12 PM   #14
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mmmmm.....Thorium....

Here is a link that explains the whole clear to yellow to clear again thing fairly well.

Activation of color centers in Glass

Who needs a science dregree now that we have google?
06-09-2008, 06:41 PM   #15
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And it grows tomatoes!

My yellowed taks are clearing up nicely using this:

AeroGarden From AeroGrow International ::: 1-800-476-9669

And I'm going to have cherry tomatoes to boot!!

Thanks for the great responses, everyone.

germar
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