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Spherical aberration
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Repeating Rifle
medicine forum Guru Wannabe


Joined: 04 May 2005
Posts: 141

PostPosted: Sat Jul 30, 2005 12:04 am    Post subject: Re: Spherical aberration Reply with quote

On 7/29/05 6:01 PM, in article
bkAGe.10956$oZ.5386@newsread2.news.atl.earthlink.net, "Mike Tyner"
<mtyner@mindspring.com> wrote:

Quote:

"Repeating Rifle" <salmonegg@sbcglobal.net> wrote

My guess is, that for most people, true aberration, especially high order
aberration, is not a big problem.

Most people experience a big dose of spherical abb when they get their eyes
dilated.

It's surprising that people with large pupils don't walk in with a special
set of problems.

-MT


I have wondered about that. Apparently, there is insufficient selective

pressure to make aspheric refractive surfaces in real eyes. Diurnal people's
vision probably has other requirements. If there is an intelligent designer,
he/she forgot about aplanatic surfaces.

Are there any nocturnal animals that do have aspheric optical systems? Do
they also have cones for central vision?

Bill
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William Stacy
medicine forum Guru


Joined: 01 May 2005
Posts: 1177

PostPosted: Sat Jul 30, 2005 1:23 am    Post subject: Re: Spherical aberration Reply with quote

Repeating Rifle wrote:
Quote:

I have wondered about that. Apparently, there is insufficient selective
pressure to make aspheric refractive surfaces in real eyes. Diurnal people's
vision probably has other requirements. If there is an intelligent designer,
he/she forgot about aplanatic surfaces.

Here we go again. No, the cornea is not spherical and is indeed
aspheric and was definitely "designed" correctly. It is precisely the
flattening in the normal peripheral cornea that corrects what would be a
horrific amount of ordinary spherical aberration under low light
conditions if we had spherical corneas. We don't, and as a result, have
very little spherical aberration.

w.stacy, o.d.
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Repeating Rifle
medicine forum Guru Wannabe


Joined: 04 May 2005
Posts: 141

PostPosted: Sat Jul 30, 2005 4:33 am    Post subject: Re: Spherical aberration Reply with quote

On 7/29/05 8:23 PM, in article
DoCGe.2721$kk6.2286@newssvr13.news.prodigy.com, "William Stacy"
<wstacy@obase.net> wrote:

Quote:
Repeating Rifle wrote:

I have wondered about that. Apparently, there is insufficient selective
pressure to make aspheric refractive surfaces in real eyes. Diurnal people's
vision probably has other requirements. If there is an intelligent designer,
he/she forgot about aplanatic surfaces.

Here we go again. No, the cornea is not spherical and is indeed
aspheric and was definitely "designed" correctly. It is precisely the
flattening in the normal peripheral cornea that corrects what would be a
horrific amount of ordinary spherical aberration under low light
conditions if we had spherical corneas. We don't, and as a result, have
very little spherical aberration.

Sorry about that. I really should not have implied that the cornea was
spheric. What I do ask is if the eye's optical system is aplanatic or
whatever the correct term is when the image surface, like the retina, is not
plane.

So I will repeat my questions in a different way. Is the human eye close to
being aplanatic because of its aspherity? I doubt it because, as stated in
earlier posts, eye performance degrades with larger pupils. Even with
perfect aplanaticity there is a loss of depth of field that might
subjectively appear to be degraded acuity.

Again, does the typical nocturnal animal such as an owl or cat have better
aplanaticity than a human?

Bill
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Dr. Leukoma
medicine forum Guru


Joined: 30 Apr 2005
Posts: 1283

PostPosted: Sat Jul 30, 2005 11:54 am    Post subject: Re: Spherical aberration Reply with quote

I've read that the trilobite had an aplanatic eye.

DrG
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William Stacy
medicine forum Guru


Joined: 01 May 2005
Posts: 1177

PostPosted: Sat Jul 30, 2005 12:18 pm    Post subject: Re: Spherical aberration Reply with quote

Repeating Rifle wrote:
Quote:

Sorry about that. I really should not have implied that the cornea was
spheric. What I do ask is if the eye's optical system is aplanatic or
whatever the correct term is when the image surface, like the retina, is not
plane.

I'm sure this has been investigated, but not by me, except that it would
make sense, since the retina is concave, the optics would correspond to
that. I will say that the region of the eye that is most concerned with
focus/defocus and image quality is the macula, which can be considered,
at a first approximation, to be flat.

Quote:

So I will repeat my questions in a different way. Is the human eye close to
being aplanatic because of its aspherity? I doubt it because, as stated in
earlier posts, eye performance degrades with larger pupils. Even with
perfect aplanaticity there is a loss of depth of field that might
subjectively appear to be degraded acuity.

I think it is, within the norms of pupil size. When you talk about huge
pupils sure, you are bound to encounter the edges of the optical part of
the cornea and get some degradation from the limbal area. That part was
not designed to be used for vision, just to make the structural
connection to the scleral. It is thicker and less transparent, so you'd
expect the image to degrade. Like opening up the F stop of a camera so
wide that you get lens edge/housing whatever in the mix.

Quote:

Again, does the typical nocturnal animal such as an owl or cat have better
aplanaticity than a human?

Yes. They have larger pupils so the optics are better in the areas they
actually use on a regular basis. We humans mostly stayed in at night
during our design process.

w.stacy, o.d.
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Repeating Rifle
medicine forum Guru Wannabe


Joined: 04 May 2005
Posts: 141

PostPosted: Sat Jul 30, 2005 8:03 pm    Post subject: Re: Spherical aberration Reply with quote

On 7/30/05 7:18 AM, in article
J_LGe.8343$_%4.4223@newssvr14.news.prodigy.com, "William Stacy"
<wstacy@obase.net> wrote:

Quote:
I think it is, within the norms of pupil size. When you talk about huge
pupils sure, you are bound to encounter the edges of the optical part of
the cornea and get some degradation from the limbal area. That part was
not designed to be used for vision, just to make the structural
connection to the scleral. It is thicker and less transparent, so you'd
expect the image to degrade. Like opening up the F stop of a camera so
wide that you get lens edge/housing whatever in the mix.

"Apodization" is a term that is probably not used much in optometry. The
transmissivity decreases ("less transparent") toward the periphery of the
pupil. It is used to avoid diffractive ring patterns near focal spots. I do
not expect that eye optical quality is good enough to where apodization cab
be useful. I do not see fringes around text on my computer monitor. The
reduction in transmissivity must serve some other evolutionary purpose. Even
in bright illumination, when pupils shrink, I never see any indication that
apodization could be useful.

Bill
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Dr. Leukoma
medicine forum Guru


Joined: 30 Apr 2005
Posts: 1283

PostPosted: Sat Jul 30, 2005 9:16 pm    Post subject: Re: Spherical aberration Reply with quote

You gotta be talking huge huge pupil for that to occur. Just my
opinion.

DrG
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