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02-12-2009, 04:44 AM   #1
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Do lens' age?

Been messing about with my old Taks.

It seems to me I remember reading that glass, given enough time, changes it's composition slowly over time. I don't mean superficial changes to the surface due to environmental factors but basic internal changes to the glass' structure and composition.

In other words glass, like everything else, is unstable and given enough time will change.

If so what's the shelf life of optical glass given the best storage conditions? Would a Zeiss Tessar from 1905 still be usable if not abused? When will my Taks start showing their age even under the best of use and storage conditions?

I'm not talking about any mechanical factors like built in shutters, diaphragms, focus mechanism etc just the glass it's self.

Anyone observe what they believe was a lens' glass suffering from old age?


Last edited by wildman; 02-12-2009 at 04:50 AM.
02-12-2009, 04:57 AM   #2
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I've seen photos taken with lenses from the very early 1900s, with the lens mounted on a DSLR via a homemade adapter. They work great, as a general rule. Of course, one has to allow for things like lack of coatings, bits of fungus and whatnot.

If anyone has anything to worry about from internal changes to the glass of your Takumars my guess is it would be your grandchildren's grandchildren, if then, and that they would just find the effect quaint and charming at the very worst.

My poor understanding of the matter is that glass is technically an extremely viscous liquid, but that it would take centuries of the effect of gravity to deform it. I've heard of measurements of ancient windows showing they are ever-so-slightly thicker on the bottom than on the top, due to gravity. But they've all been in the same orientation for centuries. Presumably your lenses get moved around a bit.
02-12-2009, 05:36 AM   #3
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QuoteOriginally posted by Mike Cash Quote
My poor understanding of the matter is that glass is technically an extremely viscous liquid
Yes that's my understanding also.

What got me to thinking about this is that optical glass is one of the most demanding and critical uses that glass is put to.

So I was thinking that , perhaps, one might start seeing changes in the results of say a series of interferometer tests if these tests were taken over time and that perhaps these changes in optical performance might occur earlier than we suppose.

I don't have a clue myself. Perhaps this is a question for the opticians at Zeiss.
I do know at this time of year I have too much time on my hands and start getting Paranoid about things I won't give a thought to come Spring.

Last edited by wildman; 02-12-2009 at 05:57 AM.
02-12-2009, 05:56 AM   #4
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[/QUOTE]But they've all been in the same orientation for centuries. Presumably your lenses get moved around a bit.[/QUOTE]


I don't know much about the specific hardness of optical glass, but I would basically assume that it is fairly hard (close to pure silica) stuff, since lenses are ground rather than cast or formed. Glass that is formed or paned needs to have extra "stuff" like lime and soda to lower the softening point, which also means it can transition at room temp to that softening "state" at a greater frequency. I have also heard that glass panes have been ever so slightly thicker at the top of cathedral windows that have been in the same orientation for centuries, mostly because the production tolerances at the time were so sloppy, it is too difficult to measure the changes from that time period without a good baseline.

That combined with the low forces on such small pieces of glass, I can't imagine them flowing very much, but again I'm no glass expert, I just piddle in glasswork occasionally.

:shrug:

dug this up online for reading, it has some mention of telescope and optical glass:
http://math.ucr.edu/home/baez/physics/General/Glass/glass.html
It is from ucr, which we all know isn't really a college. kidding!
-southy

02-12-2009, 06:06 AM   #5
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02-12-2009, 06:14 AM   #6
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The "flowing glass" is an urban myth, repeated from one book or article author to the next. I just read an article about that in a recent issue of the research magazine of Max-Planck-Gesellschaft and they completely did away with this. That old church windows show often (but by far not always!) that the stained glass pieces are thicker at the bottom end, is a simple matter of aestethics. The window makers decided to orient the glass pieces in the same way to give their works a uniform look. Glass could simply not be made to uniform thickness, as we are used to today.

So glass, does not change its form over time, even if its structure is amophous like a liquid. This definition was only chosen to distinguish the structure of glass from crystallized structures, like in metalls.

But there can be factors, which change the glass properties over time. The old Takumars with yellowing glass are a good example. Here Pentax used Lanthanum glass, which over time gets a yellow tint, due to some radioactivity.

Old lenses in my experience are much more prone to mechanical problems with the iris mechanism or the shutter, than anything else. Some early lens coatings peel off after decades, some Zeiss lenses are notorious for this.

I have an old Schneider 600mm Apo lens (LF) probably from the 1930s (so between 70 and 80 years old) and it is simply superb.

Ben
02-12-2009, 06:17 AM   #7
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QuoteOriginally posted by Mike Cash Quote
...
My poor understanding of the matter is that glass is technically an extremely viscous liquid, but that it would take centuries of the effect of gravity to deform it. I've heard of measurements of ancient windows showing they are ever-so-slightly thicker on the bottom than on the top, due to gravity. But they've all been in the same orientation for centuries. Presumably your lenses get moved around a bit.
I know some glass science.

You are correct. Regarding the "thicker at the bottom" stories, some old windows are thicker at the top more or less disputing the flow hypothesis.

While the viscous liquid model of glass is a reasonable model, glass turns out to be so viscous at room temperature that it would take about the age of the universe to get a permanent flow big enough to measure.

However there IS observable aging in many old Takumar lenses. They gradually become amber colored with time. That's due to radiation damage caused by radioactive decay of dissolved Thorium used to adjust the glass' optical properties. This amber discoloration is easily destroyed by exposing the glass to sunlight.

The amber color is due to electrons that have been whacked out of place by energetic radioactive decay particles; solar ultra violet radiation has enough energy to help move these out-of-place electrons back to where they want to be.

Dave

Last edited by newarts; 02-12-2009 at 07:35 AM.
02-12-2009, 07:31 AM   #8
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QuoteOriginally posted by newarts Quote
I know some glass science.

You are correct. Regarding the "thicker at the bottom" stories, some old windows are thicker at the top more or less disputing the flow hypothesis.

While the viscous liquid model of glass is a reasonable model, glass turns out to be so viscous at room temperature that it would take about the age of the universe to get a permanent flow big enough to measure.

However there IS observable aging in many old Takumar lenses. They gradually become amber colored with time. That's due to radiation damage caused by radioactive decay of dissolved Thorium used to adjust the glass' optical properties. This amber discoloration is easily destroyed by exposing the glass to sunlight.

The amber color is due to electrons that have been whacked out of place by energetic radioactive decay particles; solar ultra violet radiation has enough energy to help move these out-of-place electrons moving back to where they want to be.

Dave


02-12-2009, 08:00 AM   #9
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Edit: Oops I am a bit late, others have already addressed this.

QuoteOriginally posted by Mike Cash Quote
My poor understanding of the matter is that glass is technically an extremely viscous liquid, but that it would take centuries of the effect of gravity to deform it.
This is a commonly held myth. It really isn't true, or at least not to the degree that it can be measured.

QuoteQuote:
I've heard of measurements of ancient windows showing they are ever-so-slightly thicker on the bottom than on the top, due to gravity.
Yes, but consider this: the people who glazed the windows were craftsmen. At that time glass was not perfectly flat and so it was unusual for there to be any pane that was flat. Consequently, all the panes were uneven. So what do you think a craftsman would do - the current thinking is that they just aligned the panes so that the thicker end was always down. So the fact that panes tend to be thicker at the bottom is not absolute proof that the glass has flowed.

wikipedia talks about this Superfluid Glass, and Antique Glass. The examples in the last link pretty much discount the idea that glass "flows" over time.

But even it it did flow, it would take so much time that we would not notice in our short lifetimes and anyway a lens that has bee held static in one orientation for enough time for any such effect to occur is a lens that is never used, which means that no one will ever notice that its optical properties have changed?

Richard
02-12-2009, 11:48 AM   #10
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QuoteOriginally posted by newarts Quote
I know some glass science.

You are correct. Regarding the "thicker at the bottom" stories, some old windows are thicker at the top more or less disputing the flow hypothesis.

While the viscous liquid model of glass is a reasonable model, glass turns out to be so viscous at room temperature that it would take about the age of the universe to get a permanent flow big enough to measure.

However there IS observable aging in many old Takumar lenses. They gradually become amber colored with time. That's due to radiation damage caused by radioactive decay of dissolved Thorium used to adjust the glass' optical properties. This amber discoloration is easily destroyed by exposing the glass to sunlight.

The amber color is due to electrons that have been whacked out of place by energetic radioactive decay particles; solar ultra violet radiation has enough energy to help move these out-of-place electrons back to where they want to be.

Dave
^ That there is the coolest thing I've read all day. Awesome.

c[_]
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