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| Science Forum Index » Medicine - Vision Forum » spherical in eyeball lens vs. spherical in glasses... |
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| Mike Ruskai... |
 Posted: Sat Oct 31, 2009 2:48 pm |
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On or about Sat, 31 Oct 2009 04:27:14 -0700 did Salmon Egg
<SalmonEgg at (no spam) sbcglobal.net> dribble thusly:
[quote]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.
[/quote]
The one big aberration with a paraboloid is coma. But coma is much less
troublesome than SA.
[quote]
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.
[/quote]
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. |
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| Liz... |
| Posted: Sat Oct 31, 2009 9:00 pm |
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Hi Mike,
[quote]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.
[/quote]
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.
[quote]Most corneas are prolate, which offsets SA.
[/quote]
They can measure this, now.
[quote]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. 
[/quote]
It figures. I could hot-glue them to my head...
In other words, the answer is, "Not in practice".
[quote]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.
[/quote]
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 |
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| Robert Martellaro... |
| Posted: Mon Nov 02, 2009 12:28 pm |
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On Fri, 30 Oct 2009 00:06:07 -0700 (PDT), Liz <fraternobombus at (no spam) yahoo.com> wrote:
[quote]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.
[/quote]
http://www.ophmanagement.com/article.aspx?article=86679
Robert Martellaro
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Optician/Owner
Roberts Optical
Wauwatosa Wi.
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman |
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| Liz... |
| Posted: Mon Nov 02, 2009 11:47 pm |
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[quote]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.
http://www.ophmanagement.com/article.aspx?article=86679
[/quote]
Yes, this is close to what I have read too.
cheers,
Liz |
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| Salmon Egg... |
| Posted: Tue Nov 03, 2009 11:30 am |
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In article
<98690e58-3932-489c-bd72-eb26bcc07588 at (no spam) b15g2000yqd.googlegroups.com>,
Liz <fraternobombus at (no spam) yahoo.com> wrote:
[quote]http://www.ophmanagement.com/article.aspx?article=86679
[/quote]
Again I emphasize that I am not a vision professional. I have no
experience as to how people do with IOLs that correct for spherical
aberration. I think the benefits are greatly overrated. Could it be that
the purveyors of such devices are motivated by potential profit?
Most of the time, pupil size will be small enough to reduce spherical
aberration to a level where it is not significant. Only when it gets
dark will the iris open up enough to make spherical aberration a
problem. At low light levels, cones which provide acute vision and are
the receptors in the foveal region, perform poorly. There are few if any
rods in the foveal region to take advantage of spherical aberration
correction. What rods are present will be off-axis where spherical
aberration correction might actually contribute to an increase in
off-axis aberration.
I remember Oliver Sacks writing about some tribe of people with
photopsia having a complete lack of cones. In their case, rods did not
fill in where there was an absence of cones.
In short, I think we see a situation where hype is substituting for
knowledge. There is a tendency for people to want the best possible at
any price even if the improvement is minor or even imaginary.
If I am significantly wrong in my analysis, Mike Tyner is very capable
of deflating me.
Bill
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As the years go by, dying just before having to fill out a tax return has merit. |
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| Mike Ruskai... |
| Posted: Tue Nov 03, 2009 12:12 pm |
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On or about Tue, 03 Nov 2009 08:30:29 -0800 did Salmon Egg
<SalmonEgg at (no spam) sbcglobal.net> dribble thusly:
[quote]Most of the time, pupil size will be small enough to reduce spherical
aberration to a level where it is not significant. Only when it gets
dark will the iris open up enough to make spherical aberration a
problem. At low light levels, cones which provide acute vision and are
the receptors in the foveal region, perform poorly. There are few if any
[/quote]
Cones perform quite well on an illuminated street sign at night, when the
pupils are quite large. I imagine serious SA would make driving at night
considerable more annoying than not correcting it.
Or star-gazing, for that matter, where rods are useful only for averted vision
of faint fuzzies.
--
- Mike
Ignore the Python in me to send e-mail. |
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