what 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...

 

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.

 

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

Bill