spherical in eyeball lens vs. spherical in glasses

:-)

Is "spherical aberration" in the eyeball the same thing as "spherical
correction" in a glasses prescription?

e.g.
If you have cataract surgery and an IOL implanted and are left with
(I'm making this number up) +0.30 um (I think it's um)
of spherical aberration, will that look the same to you, visually, as
if you had on a pair of glasses in which the scrip was wrong by +0.30
diopters?
It won't, right? They're different, right?

Check this website (ignore the other junk on it), where it shows a
series of "E"s. Also further down, where it shows two views of green
squares.

http://www.tecnisiol.com/optics.htm

If you have the surgery, and afterwards are left with some + or -
spherical aberration in the eye (lens and cornea), can that aberration
be corrected with glasses? Or are you stuck with it?

thanks,
Liz
(easily confused by words that are the same, like "spherical" and
"spherical") - thanks....

In article
<c892c095-020f-4de7-baf9-(E-Mail Removed)>,
Liz <(E-Mail Removed)> wrote:



> Is "spherical aberration" in the eyeball the same thing as "spherical
> correction" in a glasses prescription?

No.

> If you have cataract surgery and an IOL implanted and are left with
> (I'm making this number up) +0.30 um (I think it's um)
> of spherical aberration, will that look the same to you, visually, as
> if you had on a pair of glasses in which the scrip was wrong by +0.30
> diopters?
> It won't, right? They're different, right?

It's not that simple. If you look at some letters, and they are blurry,
it could be due to a number of reasons. The eye doctor will try
different types of correction, and see which fixes your vision. They
are different, but blur is blur.

> If you have the surgery, and afterwards are left with some + or -
> spherical aberration in the eye (lens and cornea), can that aberration
> be corrected with glasses? Or are you stuck with it?

People are often told to expect the possibility of glasses after
cataract surgery. And if you go into surgery with astigmatism, and get
"straight" cataract surgery (no astigmatism correction), then you can
expect to have the same astigmatism afterwards. If you come out of the
surgery with a different amount of spherical correction than you
desired, that can be fixed. Glasses and contacts should be able to fix
both of these.

> (easily confused by words that are the same, like "spherical" and
> "spherical") - thanks....

Well, let's take another confusing word, with many meanings, like
"mental". What does that mean? Something to do with the mind. If
somebody has a mental aberration, that means that they have something
wrong with their mind. So mental must be a Bad Word. But wait! If
somebody has "mental acuity", that means their mind works well. So
maybe "mental" isn't a bad word? But "mental acuity" has an invisible
"good" in front of it. If somebody has "bad mental acuity", that means
they aren't too sharp.

I'm a layperson, so don't take the following too seriously. "Spherical"
means that the front of the eye is normally in the shape of a section of
a sphere. If the curvature is not correct (or the internal lens isn't
the right strength, or the distance between the cornea and the retina
isn't right), then your vision will be blurry, because the image is not
focused on the retina. Since, in the office, the eye doctor cannot
change the last two factors, spherical correction is applied in front of
the cornea, either glasses or contacts, so that the image is focused on
the retina. If your cornea is not spherical, that is called "spherical
aberration", and that also will cause blur. That cannot be fixed with
spherical correction, since you need different correction according to
the axis. This is called "astigmatism" (although astigmatism can be
caused by other things), and is easily fixed by glasses. Contacts are
trickier, but "toric" contacts will fix astigmatism, and I wore them for
many years.

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

I know what spherical aberration in the eye's lens is and does; Mike
explained it in earlier posts.
I know that, with too much aberration, the light from the edges of the
lens doesn't end up focused on the retina. I think it happens mostly
with light going through the edges, not through the center. ?

"A large lens without spherical aberration will usually not have
spherical surfaces."

Right. I know.

My question comes from the fact that docs can measure your cornea's
(usually positive) S.A. and choose an aspheric IOL with a
corresponding (usually negative) S.A., in order to leave you with zero
S.A. after operating. (Or so they say; I doubt the process is ever
that accurate.)

So this made me wonder how important it is to get that S.A. correction
right with the IOL.
So I wondered what the view would look like if the S.A. were off.
That's why I looked at that web site. The blur in the pictures, they
claim, is representative of spherical aberration. (Assuming
everything else in the visual system was OK.)

These pictures don't look good.
But I thought, can't they just fix this particular problem with
glasses? So I asked:

>> Is "spherical aberration" in the eyeball the same thing as "spherical correction" in a glasses prescription?

>No.

Uh oh.

In other words if you get the wrong diopter implant (which we are told
to expect some degree of inaccuracy in), then that's correctable with
glasses. But if you end up with spherical aberration in the cornea +
IOL combination, then that's not correctable with glasses.
(I am *not* talking about astigmatism.)

Then:
> Modern refractive surgery makes it possible to measure aberration and actually sculpt the eye's optical system to take most of such aberration out.

You mean cutting into the cornea?

In other words, if the IOL + cornea combination ends up having
spherical aberration after surgery, and that turns out to be
bothersome (I don't know what to expect), then the only solution would
be LASIK or RK or similar. Right?

Liz
Indy

In article
<bc3ed608-9f0d-4252-aefd-(E-Mail Removed)>,
Liz <(E-Mail Removed)> wrote:


> >> Is "spherical aberration" in the eyeball the same thing as "spherical
> >> correction" in a glasses prescription?
>
> >No.
>
> Uh oh.
>
> In other words if you get the wrong diopter implant (which we are told
> to expect some degree of inaccuracy in), then that's correctable with
> glasses. But if you end up with spherical aberration in the cornea +
> IOL combination, then that's not correctable with glasses.
> (I am *not* talking about astigmatism.)

But *I* was. Perhaps it would be best to ignore what I wrote. I don't
think we are talking about the same thing at all.

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

On or about Wed, 28 Oct 2009 23:26:52 -0700 (PDT) did Liz
<(E-Mail Removed)> dribble thusly:

>If you have the surgery, and afterwards are left with some + or -
>spherical aberration in the eye (lens and cornea), can that aberration
>be corrected with glasses? Or are you stuck with it?

Yes, it could be corrected with glasses. But not standard glasses. I don't
know off hand whether this is the kind of thing a typical optometrist is able
to offer or not. But optically, you can correct the spherical aberration of
one optical element by introducing another one with the same magnitude of
aberration, but different direction.

That's how they dealt with the error in the Hubble Space Telescope, and how
some telescope designs work fundamentally (the Schmidt camera,
Schmidt-Cassegrain telescope, Maksutov-Cassegrain, etc.).

It doesn't sound like the kind of mistake that's likely to be made, however.
--
- Mike

Ignore the Python in me to send e-mail.

>>>If you have the surgery, and afterwards are left with some + or - spherical aberration in the eye (lens and cornea), can that aberration be corrected with glasses?


>... Optically, you can correct the spherical aberration of one optical element by introducing another one with the same magnitude of aberration, but different direction.


Right. That's how those aspheric IOLs work, too. One or both
surfaces of the IOL are shaped to correct, or partly correct, the
spherical aberration in the cornea. (The S.A. in people's corneas
varies, so the degree of correction also varies in dif. aspheric
IOLs.)


> It doesn't sound like the kind of mistake that's likely to be made, however. *

It's not a mistake, exactly. It's just a result. If you implant an
IOL that has positive S.A. into an eye whose cornea also has positive
S.A., you end up with even more positive S.A., which might (???) cause
some blur or glare.

Liz
Indy
USA

>>>If you have the surgery, and afterwards are left with some + or - spherical aberration in the eye (lens and cornea), can that aberration be corrected with glasses?


> > Yes, it could be corrected with glasses. *But not standard glasses. *I don't know whether this is the kind of thing a typical optometrist is able to offer or not.


> It's the reason for "aspheric" lenses, where one or both surfaces are made
aspheric in order to reduce abberration.

I thought those glasses were meant to be less aberrant than normal
glasses, but not meant to correct an aberration in the person's eye.
True?

(I'm still not talking about astigmatism, only symmetrical S.A. I
know nobody is ever perfectly symmetrical, but I'm only asking about
the symmetrical part.)

> In routine optometry spherical abb is addressed by manufacturers who offer
> lens blanks with aspheric surfaces. There's a choice between standard and
> aspheric lenses, but the asphericity won't be custom-ground for any one
> individual.

> Contact lenses produce less spherical abb than glasses.

So.. is the answer to my question (at top) yes, or no?

thanks,
Liz

On or about Fri, 30 Oct 2009 08:24:20 -0700 did Salmon Egg
<(E-Mail Removed)> dribble thusly:

>In article <(E-Mail Removed)>,
> Mike Ruskai <(E-Mail Removed)> wrote:
>
>> On or about Wed, 28 Oct 2009 23:26:52 -0700 (PDT) did Liz
>> <(E-Mail Removed)> dribble thusly:
>>
>> >If you have the surgery, and afterwards are left with some + or -
>> >spherical aberration in the eye (lens and cornea), can that aberration
>> >be corrected with glasses? Or are you stuck with it?
>>
>> Yes, it could be corrected with glasses. But not standard glasses. I don't
>> know off hand whether this is the kind of thing a typical optometrist is able
>> to offer or not. But optically, you can correct the spherical aberration of
>> one optical element by introducing another one with the same magnitude of
>> aberration, but different direction.
>>
>> That's how they dealt with the error in the Hubble Space Telescope, and how
>> some telescope designs work fundamentally (the Schmidt camera,
>> Schmidt-Cassegrain telescope, Maksutov-Cassegrain, etc.).
>>
>> It doesn't sound like the kind of mistake that's likely to be made, however.
>
>It is not that simple. The Hubble telescope is pointed in the direction
>toward the object to be observed. In reading, for example, you swivel
>your eyes to scan a line. You do not ordinarily turn you head to scan a
>line. This means that you look through different portions of any
>corrective lenses you wear while reading. What may correct for on-axis
>reading is likely to introduce optical error when you swivel your eye to
>look through a different portion of any corrective lens.

That's true, but you're overestimating the magnitude of the complication.

Consider a tilted component telescope, which has much higher angles than the
small scanning movements of a reading eye. The image is still usable, and the
spot diagram is much smaller than it would be for straight SA.

>With refractive surgery, the cornea is sculpted to minimize total
>optical error on-axis. As you scan a line of text, the correction in
>your eyeball turns with the eye. If you had glasses that turned with
>your eyes, then aspheric correction could end up being effective no
>matter which diriction you look.

Or contacts. But I still contend that the angles are too small to make such a
hypothetical corrective lens impractical.

In reality, I expect the kind of error suggested would be corrected with
further surgery, if it ever occurred at all. If what you say is accurate,
they'd have to measure SA as normal procedure, which makes these speculative
lenses unlikely to ever be needed.
--
- Mike

Ignore the Python in me to send e-mail.

On or about Sat, 31 Oct 2009 04:27:14 -0700 did Salmon Egg
<(E-Mail Removed)> dribble thusly:

>The corresponding process in making optical telescopes is called
>"figuring." Classical figuring is used for large primary mirrors. To get
>best focussing over a small field of view, the mirror's surface was
>changed from spherical to paraboloidal. Note that it is much easier to
>fabricate high quality spherical surfaces than aspherical ones. So after
>getting a spherical surface, the mirror would be hand polished a bit to
>convert the sphere to a paraboloid. Done perfectly, it would get rid of
>spherical aberration completely. The problem is that off-axis.
>performance dropped off.

The one big aberration with a paraboloid is coma. But coma is much less
troublesome than SA.

>
>For large field applications, compromises between field-of-view had to
>be made. That is how the Hubble telescope got into trouble. Figuring of
>the mirror was incorrect for magtching thes rest of the optical train.

The problem with Hubble's mirror was incorrect figuring, period. The tool
being used to measure the curve was outside of spec, and a kludge some worker
made didn't work. It was left with considerable spherical aberration. The
intended shape was a hyperboloid, as the overall design is a Ritchey-Chretien
(as far as I know, the secondary has the correct hyperboloidal shape).

The RC design overcomes the big problem with a paraboloid - coma. RC
telescopes have no off-axis coma, trading it instead for off-axis astigmatism.
While the spot diagrams are not much smaller than those produced by coma far
off axis, they are round, which is important for astrometry.

I guess the relevant point here is that both alternates to spherical
aberration - coma and astigmatism - are far less problematic. As a
consequence, I see no problem with the idea of addressing SA preferentially in
the context of a lens implant, especially since the original lens corrects
reasonably well for it via heterogenous refraction.

But I still don't think anyone will actually be in the situation of requiring
special lenses to correct a botched surgery.
--
- Mike

Ignore the Python in me to send e-mail.

Hi Mike,

> > I thought those glasses were meant to be less aberrant than normal
> > glasses, but not meant to correct an aberration in the person's eye.
> > True?
>
> Well, yes, but SA is SA and the SA from glasses usually overwhelms the tiny
> amounts generated at the cornea or internally. And when the pupils are
> small, SA from the cornea and lens drops to nil, compared to glasses.

Yes, it seemed from what I read that the optical problems associated
with spherical aberration in the lens would only apply in dim light
when the pupil is wider.

> Most corneas are prolate, which offsets SA.

They can measure this, now.

> > So.. is the answer to my question (at top) yes, or no?
>
> You're asking whether glasses can be sculpted to create a perfect wavefront,
> like topography-driven lasers sculpt the eye? Yes, but you have to look
> through the exact center of the lens from then on.

It figures. I could hot-glue them to my head... :-)
In other words, the answer is, "Not in practice".

> Kidding aside, I haven't seen any system for designing individual spectacles
> to minimize internal SA. There may well be. But internal SA is tiny compared
> to the SA created by glasses, and almost nonexistent in bright light.

I hope that after surgery I won't wear glasses to drive.

What do you think of those blurry images on the Tecnis site?
http://www.tecnisiol.com/optics.htm

When would you ever see blur like that? Only in dim light?

Maybe contacts would do a better job of correcting SA?

thanks,
Liz