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  1. #1

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    Glass deformation

    Since glass is a fluid and gravity will eventually deform it, how long does any kind of lens have before it deforms enough to actually have a measurable (or later visible) impact on image quality? Is this something that might impact a 100 year old lens, or are we talking more like thousands of years or more? Just curious, not that this is a pracitical problem.

    Thanks

  2. #2
    edp
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    Quote Originally Posted by Michael R 1974 View Post
    Since glass is a fluid and gravity will eventually deform it
    No it's not, and no it won't.

  3. #3
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    Interesting question. I have always heard that glass is an amorphous solid as well, but I'm not sure if that's true or kind of an old wive's tale.

    Judging by all the old lenses that are still in use, I'd say it's 100% not a concern.

    Here, these are almost a thousand years old and still doing fine.... http://en.wikipedia.org/wiki/Visby_lenses (granted, they're made of quartz, but cool nonetheless)

  4. #4

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    Quote Originally Posted by edp View Post
    No it's not, and no it won't.
    Is that correct? I don't think so.

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    edp's comment about glass not being a fluid. My understanding is glass is fluid, and will "flow" over time.

  6. #6
    holmburgers's Avatar
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    The classic example given is "look at old window panes, they're thicker at the bottom!" But I don't know; it's possible that the manufacturing techniques of the day were just that imperfect, or perhaps the thickness at the bottom was for better weight-bearing.

    Why do you ask, by the way?

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    Marek Warunkiewicz's Avatar
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    The glass flowing thing is another urban legend. If it was true, you would be seeing nothing but blobs when you look at ancient Roman (and older) glass. 5,000 years old and still holding up.
    Marek Warunkiewicz

  8. #8
    edp
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    Also consider glass shelves with piles of books on, and very old, heavy mirrors in big telescopes. Shelves don't bend, and very heavy mirrors don't deform. Roman glass is as beautiful today as it was 2000 years ago, and hasn't deformed into blobs. The glass transition temperature of ordinary window glass, or of the types used in lenses, is many hundreds of degrees above room temperature.

    It's an unusual amorphous solid sometimes described as a 'supercooled liquid', and that's what causes this widespread misconception.

    Have a look at the Pitch Drop experiment, running (pun intended) at the University of Queensland since 1927. http://www.physics.uq.edu.au/physics...itchdrop.shtml

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    I rotate my lenses on my lensboards 90 degrees every 3 months just to be sure.

    Peter Gomena

  10. #10

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    Rotating the lens - funny stuff.

    I'll have to do some more reading I guess, but I don't think Roman glass is a very good example anyway. I'm not talking about such extreme deformity, but rather something merely measurable as opposed to visible. So if such deformity did occur, it surely wouldn't be visible in Roman or more ancient glass. I was just curious as to whether it would affect an optical ie: precision lens enough over a reasonable amount of time.

    It's not that I'm worried about it in my own work. I was just trying to picture classified ads hundreds of years from now for circa-2000 Schneider lenses for example, and whether gravitational deformity could be another condition factor. I mean if anyone would be concerned with such a thing, it would surely be photographers.

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