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12-06-2012, 04:03 PM   #1
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Auto Chinon MC 50mm 1.4 / 1.7, yellowing?

I've noticed that both my 50mm manual Chinon lenses (1.7 MC and Revuenon-branded 1.4 MC) have a fairly noticeable brownish tint to them. They have the highest transmission loss among all my primes by far, only some old zooms are as bad or worse. I'd chalked it up to glass or something when I only had the 1.4, but it's just as bad on the 1.7, which struck me as odd.

Now these date from the early/mid-'80s or so, and I thought that the usual yellowing related to radioactive substances was a thing of the past by then. In addition, my (AGFA-branded) Chinon 28/2.8 and 135/2.8 lenses do not exhibit anything unusual.

Any thoughts?

Unfortunately I do not have any super-high-power UV source right now (or any fluorescent lamp that I could run for days), and it's winter, so sunlight is pretty much out, too.

I'll have to clean up the 1.7 anyway, as it has very oily blades that hardly move and the aperture mechanism seems to have come unhooked somewhere, too. (Ugh.) Maybe that'll shed some light on the matter. I bet it's the last group though, at least that's what visual inspection of the aperture seems to indicate (plus it's where the problem usually seems to occur, as the two cemented elements are located there). I hope there aren't any unusual pitfalls lurking inside, that'll be my first lens to open up. Latex gloves, swabs, turpentine substitute (light mineral spirits), calipers, JIS screwdrivers, plenty of light - anything else?

12-06-2012, 04:19 PM   #2
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I haven't had the chance to test it more then in a few shots, but my Sears 1.7 (Chinon-made) seems be clear.

Out of curiosity, what tint does the multicoating seem to have? Mine has a purplish cast to it as the light hits it. I'm wondering if maybe there's something in that that's turned, as radioactivity seems unlikely.

My Chinon-Sears is the 49mm threaded 50mm in this shot, above the Ricoh (I think) Sears 50mm.

12-06-2012, 04:39 PM   #3
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My old Chinon 1.7 50 is crystal clear, held over a white sheet of paper there's no discolouration at all. Not much help I know.
12-06-2012, 08:17 PM   #4
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I recall reading some years ago that yellowing was caused by radioactive changes in the glass over time. A quick search on the internet confirmed my memory! Here are some sites:

Radioactive lenses - Camerapedia (explanation of the source of radioactivity (thorium oxide) and list of lenses known to be affected)

The Classic Camera Repair Forum: Summicron 5cm, Yellow lens (How to fix yellowing by exposure to UV light)

(Youtube demo - how to fix yellowing by exposure to UV light)

12-06-2012, 09:22 PM   #5
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I wouldn't leave a lens out in the sun for extended periods of time, ever. You are running issues like heat, casing the grease in the helicoid to separate/migrate etc. If you want to test if the browning is due to radioactivity you could simply try to de-yellow them with something like this.
Amazon.com: Ikea Black Jansjo Desk Work Led Lamp Light: Home Improvement
Ive read on forums of people using that particular lamp. I think any LED lamp would work fine, however once that concentrate the beam (like the one above) work better. So wrap it aluminum foil as in the video above and leave it over-night under the lamp. If the brown tint goes away then it was caused by thorium. If doesnt go away, its not radioactive and something else is happening.
12-06-2012, 09:41 PM   #6
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QuoteOriginally posted by PJ1 Quote
yellowing was caused by radioactive changes in the glass over time
...though radiation is not the only possible cause. The yellow is due to chemical changes in the balsam cement that holds the multi-element groups together.

That being said, the list of radioactive lenses seems to be growing steadily as various lenses are tested. Here is a link to an image and lengthy discussion/listing of "hot" lenses.

Radioactive lenses -- group shot | Flickr - Photo Sharing!


Steve
12-07-2012, 09:05 AM   #7
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QuoteOriginally posted by 52mm Quote
I've noticed that both my 50mm manual Chinon lenses (1.7 MC and Revuenon-branded 1.4 MC) have a fairly noticeable brownish tint to them. They have the highest transmission loss among all my primes by far, only some old zooms are as bad or worse. I'd chalked it up to glass or something when I only had the 1.4, but it's just as bad on the 1.7, which struck me as odd.
My Chinon 50 1.4 MC is clear.
QuoteOriginally posted by 52mm Quote
I hope there aren't any unusual pitfalls lurking inside, that'll be my first lens to open up. Latex gloves, swabs, turpentine substitute (light mineral spirits), calipers, JIS screwdrivers, plenty of light - anything else?
A large shallow plastic tray lined with some paper towel to work in and to catch any errant bits. Be methodical, step at a time, make notes or take shots of where and which way round thing were. Mark the positions of everything before you unscrew it. Don't disassemble anything you don't need to. Have fun
12-08-2012, 03:03 PM   #8
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QuoteOriginally posted by stevebrot Quote
...though radiation is not the only possible cause. The yellow is due to chemical changes in the balsam cement that holds the multi-element groups together.
The radioactive glass is definitely the main the source of the yellowing.

12-08-2012, 07:59 PM   #9
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Thanks for the link. I had never seen any example where individual non-cemented elements had been shown as colored and had been cleared by UV light. That being said...Yellowing, by itself, it is still not proof positive of radiation. It may instead, simply be an indication of inferior balsam used in assembly that as yellowed with age or perhaps even mildly tinted glass used in the original construction.


Steve

Last edited by stevebrot; 12-08-2012 at 08:05 PM.
12-14-2012, 01:10 PM   #10
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So it appears it's not a general problem for these Chinons. Interesting.

Here's what my Chinon 50/1.7 (S/N C638519) thinks is a white sheet of paper, complete with messed-up aperture:


By contrast, an A 50/1.7:


And an M 50/1.7:


And for kicks, a single-coated mid-'70s Porst (Cosina) M42 affair (6 elements / 4 groups, very Biotar-ish bokeh):


I'd say if you're falling way behind a single-coated lens in transmission, you've got a problem.

Now for the 1.4s.
Revuenon (Chinon OEM) 50/1.4, S/N C627455:

- vs. -
XR Rikenon 50/1.4:


Finally, a shot of coating reflections, Rikenon P vs. Chinon 50/1.7:


Those should correspond to the bottom and top Auto Sears in #2, respectively. (The Rikenon P actually happens to be among my very clearest lenses, btw, at least on par with the A 50/1.7.) My two Chinons (1.7 and 1.4) seem to share the same kind of coatings.

(All shot with AGFA-labeled Chinon MC 28/2.8 @ f/5.6, manual WB.)

Last edited by 52mm; 12-14-2012 at 01:52 PM.
12-14-2012, 08:24 PM   #11
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Maybe it is the monitor calibration on my laptop, but none of the above lenses look particularly yellowed.


Steve
12-16-2012, 06:59 AM   #12
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Well, they probably aren't "bad bad" like some radioactive lenses can be, but the two Chinons definitely don't qualify as "good". (Incidentally, the f/1.4 looks just as yellow as the f/1.7 from behind.)
Here's how they stack up in my little array of lenses. The only one with significantly more light loss is the Tokina AT-X 28-85 (Close Focus), a far more complex lens. That kinda doesn't seem right, especially when transmission on the 28mm and 135mm is just fine (I guess those may not have any cemented elements).
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