target refraction

(re: How to choose what focal distance to ask for in cataract
surgery.)



>>>Diopters are distance so you can confidently say your refraction is about -1.00.


>>If my scrip for distance is -0.5D spherical, with 0.5D cylinder at 55 degrees ........ then is my "focal distance" -1? or -0.5?


>[Both.] It's -1.00 in one meridian and -0.50 in the other [90 degrees away].


>........Most of these calculations are done by averaging the astigmatism into a single-number "spherical equivalent" of
-0.75. So, if you want to match the first eye exactly, you'd specify
-0.75.


>> I do....[want to match the first eye exactly].


> You are probably overestimating the importance of having both eyes exactly equal. ..... When you have no accommodation, a little inequality is helpful.
If it were -1.00, I'd target -0.50 myself.... [because]... I'd be
trying to choose what is most functional WITHOUT glasses. For me,
that'd be about -050 in one eye and -100 in the other.


Ah! Got it. Then you would have a different goal than I do.


I see the tradeoff.
For myself, I'd prefer both eyes the same focal distance:


a) The current focal distance seems to be fine - I can drive and read
with modest correction, and can see most things around me well enough
to function without glasses on. If I have to be monofocal, this is a
good focal distance for me.


b) I'm eager to have stereo depth perception again. (I still lack
it after the operation, because the other eye has now become too
blurry to get good images.)


c) I'm already used to having both eyes focus together.


>We use the spherical equivalent concept to "round out" the astigmatism for purposes of matching the other eye.


OK. So, if you were to shoot for "identicalness"...
You'd measure the corneal astigmatism of the second eye, subtract that
from ...

-1D ?

or -0.75D?


And target the result as the spherical.... right?


thanks,
Liz
Indianapolis USA

I........Most of these calculations are done by averaging the
astigmatism into a single-number "spherical equivalent" of -0.75.

Wait, how did you do that?

Is it
spherical + cylinder - 1/2[cylinder]?

so
0.50 + 0.50 - 1/2[0.5]? Something doesn't look right about that.

So if someone were -2.50 +0.50,
then their "spherical equivalent" would be.... oh dear. -2.75,
because they're
-3.00 in one axis and -2.50 in the other?

Liz

> > So if someone were -2.50 +0.50,
> > then their "spherical equivalent" would be.... oh dear. * -2.75,
> > because they're
> > -3.00 in one axis and -2.50 in the other?
>
> Almost. Except when you add +0.50 to -2.50, you get -2.00.
>
> Using that, -2.25 is the right answer.
>
> That's the concept of spherical equivalent.


Is the cylinder always a positive number?

So that person, who's -2.50 +0.50, would be
-2.50 in one axis and -2.00 in the other?

That's the part that's confusing me, is which way the cylinder goes,
pos or neg.

Is there ever a scrip such as "-2.50 -0.50"? Or is it always "+0.50"?

thanks,
Liz

In article
<bcdc4104-3bd9-41c9-b99c-(E-Mail Removed)>,
Liz <(E-Mail Removed)> wrote:

> > > So if someone were -2.50 +0.50,
> > > then their "spherical equivalent" would be.... oh dear. * -2.75,
> > > because they're
> > > -3.00 in one axis and -2.50 in the other?
> >
> > Almost. Except when you add +0.50 to -2.50, you get -2.00.
> >
> > Using that, -2.25 is the right answer.
> >
> > That's the concept of spherical equivalent.
>
>
> Is the cylinder always a positive number?
>
> So that person, who's -2.50 +0.50, would be
> -2.50 in one axis and -2.00 in the other?
>
> That's the part that's confusing me, is which way the cylinder goes,
> pos or neg.
>
> Is there ever a scrip such as "-2.50 -0.50"? Or is it always "+0.50"?

I don't really want to go there, because I always get horribly confused,
but yes, there is both plus and minus cylinder, but I think not on the
same machine. ODs use one, MDs use the other. The conversion is not
obvious.

--
Dan Abel
Petaluma, California USA
(E-Mail Removed)

> Yes, -250-050 can be the same as -300+050.

COOOOL, Mike, I think I got it now.

>.....The only difference is which one you choose to mention first.


OK, try these examples: :-)



Dick's refraction is -3.00 sph -1.00 cyl (ignore the axis).

Thus, one of his meridians is -3.00. The other meridian is -4.00.

His "spherical equivalent" is -3.50.

Right?


Jane's refraction is +2.00 sph +0.50 cyl.

One of her meridians is +2.00. The other is +2.50.

Her spherical equivalent is midway between, at +2.25.

Right?

Liz's refraction is -0.50 sph -0.50 cyl.

Liz's scrip could also be written as

-1.00 sph +0.50 cyl

Right?

I hope so.


One thing puzzles me. I tested my spherical equivalent by
demonstration, by looking at that book cover without glasses, and
measuring the distance (it was 1 meter).


So why didn't my scrip turn out to be two numbers that average out to
-1.00, in agreement with what I saw?
Why did it instead turn out to be two numbers that average out to
-0.75?


(I feel SO much better when this all makes sense! Thanks......)


cheers,
Liz
Indy

On Jun 3, 1:46*am, Liz <fraternobom...@yahoo.com> wrote:

> One thing puzzles me. * I tested my spherical equivalent by
> demonstration, by looking at that book cover without glasses, and
> measuring the distance (it was 1 meter).
>
> So why didn't my scrip turn out to be two numbers that average out to
> -1.00, in agreement with what I saw?
> Why did it instead turn out to be two numbers that average out to
> -0.75?

Mainly because estimating refractive error by measuring distance to
the far point is not very accurate and does only provide an estimate.
Getting it within 0.25D is better than one would expect.

Judy