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Thread: strange lens phenomenon

  1. #1

    Join Date
    Jul 2008
    Location
    London
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    114

    strange lens phenomenon

    sorry if this is in the wrong place but this seemed most appropriate

    I wear glasses and without them nothing is in focus from my nose to infinity
    they are fixed focus not vary focals or the like

    yesterday i noticed a neon sign reflected on the inside surface of my glasses and it was tack sharp

    at first I thought i could focus onto that surface that would be about 10mm from my eye but holding a phone up without my glasses did not work
    then I thought that i am focussing at infinity via the reflex of the lens but taking my glasses off and looking at the sign it was very blurred

    maybe i’m focusing on the inside of the front surface so focussing thru the lens ?
    but then surely i would be then focusing back thru the lens so effectively looking thru the lens twice
    which would at least double the image size or something but the sign was the same size as it was when i looked at it straight with my glasses

    anybody know what is going on ?

    best

    robin

  2. #2

    Join Date
    Jul 2016
    Location
    Clearwater, FL
    Posts
    23

    Re: strange lens phenonomen


  3. #3
    ic-racer's Avatar
    Join Date
    Feb 2007
    Posts
    6,736

    Re: strange lens phenomenon

    You really want to see the ground glass up close. Have you tried a loupe?

  4. #4

    Join Date
    Jul 2008
    Location
    London
    Posts
    114

    Re: strange lens phenomenon

    hi Jeff and Ic-racer
    I should clarify

    when I say glasses I'm referring to eyeglasses aka spectacles and not the ground glass focusing screen

    robin

  5. #5

    Re: strange lens phenomenon

    The inside surface of your spectacle lens is not flat, it will be contoured to your prescription. Is it focusing the reflection to accomodate your eyesight?
    See if a flat piece of glass (maybe a filter) does the same thing and report back- that would really be revealing.

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