Should a Lens be Baked or Broiled?

Started Jul 9, 2011 | Discussions thread
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Joebobb
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Should a Lens be Baked or Broiled?
Jul 9, 2011

Hello all,

I expect to get some entertaining replies to this thread, but the question I ask is in earnest...

I decided to play around with manual lenses on my E-620 and over the course of three weeks ended up with six different lenses in total...none of which cost me much money at all - although none are in pristine condition either. Regardless, they have all surprised me. Some are remarkably good (for example, the Komine-made Vivitar 135mm f2.8). Some were remarkably bad (such as the Komine-made Vivitar 28mm f2.8). One of the lenses I bought was a Vivitar Series 1 28-105 f2.8-3.8 (made by Cosina). I paid 10 € for it (they seem to go for about 100+ € at fleaBay). Unfortunately, the first shots I took with it had a horrible, yellow tint. After doing a little research on-line, I came to find out that this is caused by "browning" of certain lenses within the telezoom due to exposure to gamma radiation being emitted from a rare-earth element contained therein. Other than the browning, the lens is in very good condition.

I've read various comments about how to reverse this (it is apparently a metastable transformation). Some anecdotal commentary suggested that it could be reversed via UV exposure over a considerable length of time (e.g. 30 days or so of direct sunlight). However, another article I read suggested that heating of the glass element was really the manner in which this phenomenon is reversed; since it has to do with a re-ordering of the electrons within the glass' atomic structure itself). The PDF at this site describes the chemistry behind the process in layman's terms...

http://www.isomedix.com/uploadedFiles/TechTeam/09%20Radiation%20Processing%20for%20Glass%20Coloration,%20Discoloration.pdf

I've had the lens in the sun now for a couple days, but it has occurred to me that if heat is the primary tool for the correction of this issue, then exposure to heat for some period of time would be the best route to reversing the discoloration. The aforementioned article describes holding the glass at its annealing temperature (roughly 300+ C) for about three hours. The article also mentions (and this is the key point here) that repeated or prolonged exposure to even lower heat (dishwasher cycles are referred too) can reverse the coloring.

I do not want to disassemble this lens in order to access the discolored, inner elements. But I wonder if it is possible to subject the lens in its entirety to some elevated temperature for a long enough period of time to reverse the process, without damaging the lens? Obviously, a lens such as this can withstand extended temperatures up to some threshold without damage. The question is, what is this threshold?

I found some commentary elsewhere suggesting that 55C is a threshold for fluorite glass elements (apparently one of the reasons Canon L lenses are light beige in color). Since this old Vivitar probably contains no plastic parts, I think the greatest danger that elevated temperatures would pose would be either a) a breakdown of internal oils or greases, causing degraded performance and possibly oil migration in the lens - including possible migration or deposition on internal optical surfaces) - or - b) heat damage to the outer, rubber grip surface.

My thoughts are that I could try an exposure of, say 24 hours at 50C (122F) in the oven, followed by a gradual cool-down to room temperature (leave lens in oven after turning off; allowing both to descend to room temperature as the oven dissipates stored heat energy).

Any comments? Bear in mind that what is at stake here is a lens that cost me all of 10 €...so if I break it, it's no great loss.

Kind regards,

JoeBobb

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