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06-26-2012, 09:45 AM   #1
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Takumar yellowing question

So I've got like 4 SMC/Super-Multi-Coated Takumar 50 1.4s, and one SMC Tak 55/1.8 that all look *very* slightly yellowed when looking through the lens (off-camera) at a white background. It is so slight that I probably wouldn't notice it, except when I compare them to other lenses they clearly have a bit of relative warm tone. I've already given them the UV treatment -- I think a couple of them were slightly more yellowed than they are now, but no more improvement seems to be forthcoming. Or maybe I just need to be more patient.

So my question is do these lenses EVER look totally white white neutral when looking through, or do the coatings just give a bit of warm tone? If I take an SMC Pentax-M 50 1.4 or 1.7 and compare it side-by-side to these Taks, the Ms look perfectly neutral (by which I mean the white background looks the same through the lens and not through the lens) while the Taks look slightly warm in comparison. And I also have an old 8-element Super-Tak, and that one actually has a slighly cool cast when looking through -- the white background looks slightly bluish.

So what's going on with the Taks? Do they need more de-yellowing until they look as neutral as the Ms, or will they never look like that? All of them look exactly the same now -- none is more yellow than another, which makes me suspect that's just the way they are, but I don't know.

06-26-2012, 10:53 AM   #2
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How exactly did you do the UV treatment? If you put it near a window it might not work or work very slowly, a lot of modern windows have a UV coating and will block a majority of the rays necessary to clear up the yellowing. I used a fluorescent UV light to clear up my 50mm f1.4 and after 24 hours there was a significant improvement. If you use a UV bulb you need to be sure to find a real one, not a "party" black light.
06-26-2012, 11:05 AM   #3
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QuoteOriginally posted by elliott Quote
How exactly did you do the UV treatment? If you put it near a window it might not work or work very slowly, a lot of modern windows have a UV coating and will block a majority of the rays necessary to clear up the yellowing. I used a fluorescent UV light to clear up my 50mm f1.4 and after 24 hours there was a significant improvement. If you use a UV bulb you need to be sure to find a real one, not a "party" black light.
I got a Repti-Glo reptile UV light, put it in a little trapazoidal lamp (small accent lamp with paper shade) and had it upside down almost touching the lens and I covered the whole interior of the lamp with foil and with foil underneath it all. I actually found I could fit more lenses in the corners so I actually had all 5 of them in there at once -- 4 in the corners angled toward the light and one directly below the light in the center. I rotated the positions once in a while but they were all in there at least 4 days. Like I said, I think I did see some improvement in some of them, but none were really bad to begin with. And they all look identical now, but still clearly warmer than the M 50 1.4 & 1.7, which are absolutely white neutral.

So I then I saw a link on this forum somewhere that said a Jansjo LED task lamp from IKEA would clear a lens up faster than anything (12 hours for a bad case was claimed). I was somewhat dubious as I am unaware of LEDs putting out significant UV light, but then I'm unaware of a lot of things and I needed a little desk lamp anyway. So I went to IKEA yesterday and picked one up for $10 and jammed the light right into the lens and left it overnight. This morning it looks exactly the same.
06-26-2012, 12:04 PM   #4
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I just realized I have another lens that I have not treated because it has been in a state of disassembly, but looking at it I see it is more yellowed than any of the others. So if I give it the treatment and it improves that will at least tell me the lights are doing something...

06-26-2012, 12:37 PM   #5
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Window glass and glass used in common table lamps block UV. Putting a yellowed lens inside, behind a window, or under a table lamp, will not cure the yellowing, or will take a loonnngggg time.

What I did to my yellowed Takumar was to wrap it in aluminum foil, except for the top, (so that the sunlight reflecting back to the lens) and put it in the sun in a cold day (heat may cause the lubricant in the lens to melt, contaminating the aperture blades). It took 2 days to cure the yellowing.

I think an EPROM eraser will work nicely. Or if you have a friendly tanning salon in the neighborhood, they may be willing to help....
06-26-2012, 12:43 PM   #6
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QuoteOriginally posted by SOldBear Quote
Window glass and glass used in common table lamps block UV. Putting a yellowed lens inside, behind a window, or under a table lamp, will not cure the yellowing, or will take a loonnngggg time.

What I did to my yellowed Takumar was to wrap it in aluminum foil, except for the top, (so that the sunlight reflecting back to the lens) and put it in the sun in a cold day (heat may cause the lubricant in the lens to melt, contaminating the aperture blades). It took 2 days to cure the yellowing.

I think an EPROM eraser will work nicely. Or if you have a friendly tanning salon in the neighborhood, they may be willing to help....
Both of the lamps/bulbs I'm using were recommended for this, so we'll see. This other lens that I just stuck under the IKEA lamp is pretty obviously yellow, so it should be just as obvious if it goes away. The other ones are pretty subtle, so I'm just trying to figure out if I've got a little more to go or I'm just obsessing...
06-26-2012, 01:03 PM   #7
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I did a similar deyellowing action some weeks ago, with a cheap 4W blacklight.
One week did the job.
An earlier attempt a few years ago with an eprom eraser did not show much effect.
10-26-2016, 05:38 AM   #8
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Sorry for bumping a very old thread, but I still don't think the original question was fully answered.

I have a Super Takumar 50mm F1.4 that was quite yellowed when I got it. I picked up an IKEA lamp, as others have recommended. After 48 hours the yellow tint was greatly reduced. I soon discovered, however, that the longer the process continues, the slower it goes. I've had it for a couple of weeks now and kept it under the lamp for much of that time, and it still has a subtle, slightly yellow or "warm" tint. Will it ever clear up completely?

I think the lens is usable now with auto white balance or with a slight adjustment in Lightroom. However. . . I have other lenses that don't require any correction, so why would I grab this one when I leave the house?

10-26-2016, 09:05 AM   #9
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It will probably never get to pure neutral white. To judge these, I put a white paper on the wall and hold the lens up wide-open looking through it so I can see how the white differs between the lens and not through the lens. Do that with other (non-yellowing) lenses and you'll see that although some lenses do seem totally neutral and the paper looks the exact same color with/without, many lenses have a cast of some type (later Pentax K & M lenses tend to have a bluish cool cast, for instance) and that's normal for that lens. So is the slight yellow leftover just the way the are? (They way they originally were decades ago.) Or is it something else (like the cement yellowing) which is not going to get fixed? I don't know, but once it gets to the point where you aren't seeing any more change, and as long as it is only slight (and even), then I think you'll find that is good enough and the lens will perform well.

Whereas a heavily yellowed lens actually not only has a cast, but is less sharp.

I'd like to see a before and after lens IQ test, but I always forget. So let me just say to anybody reading this thread that has a yellowed lens that they haven't treated yet -- take some pictures of it and WITH it now -- pictures you can reproduce under the same conditions later. And then after it is de-yellowed we can hopefully see the difference not only in the yellowing itself but the sharpness & color fidelity improvement.
11-10-2016, 01:19 PM - 1 Like   #10
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Hi, I hope you don't mind me jumping in this late with some related input.
I tried clearing an SMC Tak a year or two ago using the Ikea lamp and it worked fairly well though there is still a tiny bit of yellowing left.
A week or so ago, I bought another one (Super Tak this time) which was much more yellowed so I went on Ebay and bought
a pair of germicidal UV tubes for use inside pond filters and set them up in a lamp box I have.

They seem to be quite effective but one thing I underestimated was the power/danger of these things and I hope this serves
as a warning for anyone else who might otherwise blunder the way I did:
I didn't wear eye protection when I was positioning the lenses next to the tubes and the following day my eyes were stinging
like crazy, gushing like taps, I had some visual impairment and had to pay a visit to hospital.
What I had sustained were ocular flash burns but luckily the damage I'd done to my eyes was temporary and cleared up after three days.
Don't let this happen, it is VERY nasty, the lamps emit short-wavelength UV-C which is so hazardous it can also cause skin burns.

Soooooo ...the Super Tak is back beside the lamp which is now covered by a cardboard 'cowl' and the UV glasses are poised on top!
I'll check back in with the end result.
11-10-2016, 02:02 PM - 1 Like   #11
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QuoteOriginally posted by Photator Quote
I didn't wear eye protection when I was positioning the lenses next to the tubes and the following day my eyes were stinging
like crazy, gushing like taps, I had some visual impairment and had to pay a visit to hospital.
What I had sustained were ocular flash burns but luckily the damage I'd done to my eyes was temporary and cleared up after three days.
Don't let this happen, it is VERY nasty, the lamps emit short-wavelength UV-C which is so hazardous it can also cause skin burns.


Thanks for posting this warning!

...Oh, and welcome to the Pentax Forums!


Steve
11-10-2016, 03:03 PM   #12
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I might as well give an update too. . .

I continued to treat my lens under the Ikea lamp, and then for good measure I gave it a few hours in the Texas sun on a not-too-warm day. Although there still appears to be a slight tint when peering through the lens, some test photos showed that the effect is negligible bordering on undetectable with photos. So, yes, it's possible to get a lens fully clear, or so close as to make no difference.
11-10-2016, 03:57 PM   #13
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Another warning, probably not necessary but hey: I hope everyone is looking through their lenses from a few inches away. The radioactivity is low but there is AFAIU some risk from regular close eye exposure.
I read that when thorium was used in the eyepieces of cameras, people were getting cataracts from them.
11-10-2016, 11:12 PM   #14
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QuoteOriginally posted by Photator Quote
I read that when thorium was used in the eyepieces of cameras, people were getting cataracts from them.
Gunsights, spotting scopes, and binoculars.


Steve
11-11-2016, 04:12 PM   #15
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OK, I've got a correction to make. When I said the yellowing is now "negligible", I was looking at a couple of film scans. The guys who did the scans apparently ran color correction on them, which I should have guessed, which made them look almost identical. Repeating the same test with my Sony A7, with a fixed value for white balance, then the Takumar comes out with a distinctly yellow tint. When I correct both images with custom white balance (using a white target), then they come out looking identical.

So, maybe it's really not practical to clear the yellow 100% after all, but it's easily fixed.
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