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Medical Forum / General / Vision / October 2006what are base down prisms for?
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? 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 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... > 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 > 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 [quoted text clipped - 15 lines] > is keeping me from needing glasses all the time then it seems like > over-focusing is not a bad thing... I think your optician misread the prism direction; ask her to call the doctor and verify by phone. If you were overfocusing and eyes were over converging then you need base OUT prism in each eye, not base down. The "O" was misread as a "D" Dr Judy > > 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 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. > > 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 [quoted text clipped - 29 lines] > > > > > > dr grant How does a prescription with prism differ in principle from that with decentered lenses? Is there more to it than mere notation? Bill |
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