what are base down prisms for?

Discussion in 'Optometry Archives' started by arris66@yahoo.com, Oct 1, 2006.

  1. Guest

    I am 40 and I'm a little farsighted and have had glasses for reading
    since I was about 25. I have had more eyestrain than usual lately when
    doing close things, so visited the optometrist, who told me that my
    prescription is mostly the same, except she took out the astigmatism
    part that used to be in one lens, and she was adding prisms, which I
    didn't have before, because my eyes converge too much and she said this
    should make the light focus upward which would make my eyes converge
    less.
    When I took the prescription to another place to pick out glasses the
    optician there said that "base down" prisms aren't supposed to be
    prescribed in pairs, that if one eye has "base down" then the other
    should have "base up", otherwise they are cancelling each other out and
    doing nothing. He implied that since my prescription is at present the
    same in both eyes and has no astigmatism that the first optometrist was
    possibly only trying to do something to write the prescription that
    would make it so that I couldn't just buy off-the-rack drugstore
    reading glasses.
    Is this true? Is there any benefit to the prescription I was given?
     
    , Oct 1, 2006
    #1
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  2. CatmanX Guest

    Yoked prism is prescribed to trick your brain into thinking it is
    looking further away. You still need to converge just as much, but
    there is less strain on the eyes. It does work and I wear 3^base down
    in my glasses for what it is worth.

    dr grant
     
    CatmanX, Oct 1, 2006
    #2
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  3. Guest

    Thanks much for the reply. Ok, I already ordered the glasses
    (regardless of what the 2nd guy said) so I'll see how it works when I
    get them...

    But how does it work? I mean, why does it trick the brain into
    thinking it's looking further away?
    And will that be disorienting - like make me feel like I am taller? Or
    shorter? I had that problem with some glasses a few years ago but I
    think it was because of the prescription for astigmatism - if I walked
    around with them on they made me feel short, and they made things like
    trees look warped. But that went away after about a week.

    One other question: the eyedoctor said that I "should" be able to see
    at all distances with my glasses, but that the reason my distance
    vision is blurry with glasses is because I over-focus. She didn't tell
    me any reason for this or whether it is good or bad. Is there a way
    not to over-focus? And if there is, is it a good thing? I mean, if it
    is keeping me from needing glasses all the time then it seems like
    over-focusing is not a bad thing...
     
    , Oct 2, 2006
    #3
  4. Guest

    They did check and the eyedoctor did confirm that it is supposed to be
    base down in both lenses. Apparently it is for the reasons that the
    other poster explained - some way to trick the eyes (brain?) into
    thinking that they aren't looking at something as close. I guess I'll
    see how well it works.
     
    , Oct 3, 2006
    #4
  5. Salmon Egg Guest

    How does a prescription with prism differ in principle from that with
    decentered lenses? Is there more to it than mere notation?

    Bill
     
    Salmon Egg, Oct 4, 2006
    #5
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