Eye See

[ChrisGreaves]

2023_09_20230919_150635.jpg

Above: My right eye

2023_09_20230919_150647.jpg

Above: My left eye.

Both images represent what opthamologists see when they shine bright lights in my face; head-on.

Don't panic. That's not my finger over the lens (as often it is), nor is it a malignant eye.
Brittany told me what it was, something to do with the imaging system, but I have forgotten.

cranialnervesanatomy.jpg

This schematic image modified from https://biologydictionary.net/wp-conten ... natomy.jpg shows the optic nerve leaving the eye from the centre of the retina, then joining/crossing the other optic nerve.
So many diagrams are schematic, and all my life I have thought of that optic nerve as leaving dead-centre.

I would expect evolution to have favoured faster recognition, which generally means a shorter path. So animals whose optic nerve was closer to the centre of the skull would have a slight but measurable advantage.

Of course I would also have expected evolution to favour a direct path, straight-back to the brain. I mean, why not swap the two eyeballs for an even shorter path?

And yes, I had considered posting this in the Cooking Forum under "Soft-boiled eggs".
Cheers, Chris

[HansV]

Why do both your eyes have a little notch on the upper right side?

[ChrisGreaves]

Why do both your eyes have a little notch on the upper right side?

Hans, I don't really know. It doesn't look notchural, does it?
I'll ask Brittany or Tammy when I go to collect my new specs.
¡Buenos Noches!, Chris

[stuck]

And to ensure this topic is in keeping with in the spirit of all good Scuttlebutt topics, I'll digress and mention that today I took delivery of an 'i1', pronounced 'eye one'.

Ken

[kdock]

Hans,

Why do both your eyes have a little notch on the upper right side?

Those are the handles.

You're welcome.

Kim

[HansV]

The handles we use to roll our eyes?

[John Gray]

Chris: I would suggest that you immediately make haste to an eye-specialist, for your eyes are significantly distorted in the horizontal and vertical dimensions. Perhaps you will be instructed to rotate your head rapidly, to make the eyes more circular. Or maybe spectacles lenses could be cut to alleviate the obvious distortions. I doubt any firm other than Zeiss would be able to do this. Prepare for a bill of $1k to $5k.

[ChrisGreaves]

Chris: I would suggest that you immediately make haste to an eye-specialist, for your eyes are significantly distorted in the horizontal and vertical dimensions. Perhaps you will be instructed to rotate your head rapidly, to make the eyes more circular. Or maybe spectacles lenses could be cut to alleviate the obvious distortions. I doubt any firm other than Zeiss would be able to do this. Prepare for a bill of $1k to $5k.

John, I was AT the eye-specialist which is how I came to obtain the images.
The distortion is due to Brittany not vacating the chair to allow me to sit with my camera directly in front of the screen; I had to lean in, diagonally as it were, to take the photo,
There are time limits on how long an old man is permitted to lean over the shoulder of a young girl sitting on a chair.

The computer (Big Black Box on the floor accumulating carpet dust) "is not", she says. "connected to the internet", but I suspect it must surely be connected to some other computer in the office that IS connected to the internet.

I am getting special lenses cut. One weighs in at 5.37 and the other at 4.25, which, for the uninitiated, means that I am now moving well outside the orbit of those el-cheapos in the rack at the pharmacy.
Cheers, Chris

[kdock]

The handles we use to roll our eyes?

You got it in one!
K

[John Gray]

I am getting special lenses cut. One weighs in at 5.37 and the other at 4.25, which, for the uninitiated, means that I am now moving well outside the orbit of those el-cheapos in the rack at the pharmacy.

Pshaw! I must needs inform you that before I had my cataracts done a couple of years ago, both my eyes "weighed in" at about -15 dioptres...

[HansV]

-15? Wow!

[ChrisGreaves]

I am getting special lenses cut. One weighs in at 5.37 and the other at 4.25, which, for the uninitiated, means that I am now moving well outside the orbit of those el-cheapos in the rack at the pharmacy.

Pshaw! I must needs inform you that before I had my cataracts done a couple of years ago, both my eyes "weighed in" at about -15 dioptres...

dioptres?

If I understand so far:-
(1) A Diopter is a measurement of focus strength
(2) There are two lenses in each eye: Corneal and Crystalline
(3) Focus assumed at 1/60th metre which is 1.6cm (which seems about right for the depth of an eyeball)
(4) My lenses are prescribed at 5.37 and 4.25
(5) Which suggests 45 + 15 – 5.37 and 45 + 15 – 4.25
(6) So that my crystalline lenses have degraded to 9.63 and 10.75 respectively.
Your statement that you weighed in at -15 suggests that your eyes are in very good shape, and that you do not need or use reading glasses. Is that close?
As regards cataracts, I am being scheduled for cataract surgery. Since the wait time is up to three years for poor people, “book now” says the ophthalmologist.
In Ontario cataracts are a bit of a scam. Back in the late 1990s the Ontario provincial government in its infinite wisdom decided to save money by eliminating the supposedly-free eye check every two years and decreed that only folks with cataracts could go on getting free eye checks,
So it was that my then-optometrist announced “Good news! You have cataracts!!” and when I came around she explained that this meant that I could go on getting free eye exams every two years.
Turns out that about 80% of the Ontario population have cataracts, if the optometrist has a microscope powerful enough.

And if the optometrist doesn’t have such a powerful microscope, they’ll be able to afford one after another month’s billing!
Cheers, Chris

[ChrisGreaves]

-15? Wow!

Hans, swivel your monitor to face the western window and John will be able to tell you its contents.
I think
Cheers, Chris

[John Gray]

Your statement that you weighed in at -15 suggests that your eyes are in very good shape, and that you do not need or use reading glasses. Is that close?

Well... almost the exact opposite!
"-15 dioptres" means extremely (perhaps even exceptionally) short-sighted. Without my glasses I could see in focus from the tip of my nose to about 5cm away - excellent for looking at very small, close, brightly-lit objects, but pretty disastrous for anything else!

Without the cataracts and with the lenses replaced with plastic ones inside the eyes, the aim was that my vision would end up at ≈0 dioptres. After a couple of years I have gone to about -2 to -3 in both eyes, which is good enough. I still use varifocals, but could theoretically take them off for/while driving, since my long-distance vision is now 'almost' normal.

[ChrisGreaves]

Well... almost the exact opposite!

Not for the first time.
OK John, I shall go back and re-study the diopter business and find out where I wandered off the path. I have a batch of questions to ask Tammy when I go to collect my new goggles.
Cheers, Chris

[jonwallace]

The big problem, of course, is that human eyeballs are just crap. Have a squint at this video... https://www.youtube.com/watch?v=fPnYH06VJVo

Oh, and don't talk to me about eyelashes or corneal erosions!

[ChrisGreaves]

Well... almost the exact opposite!

OK, I went back and re-read your post, my link, and wrote it out again long-hand.
One divided by Diopter gives Focal length.
One Diopter is defined as a focal length of one metre.
So two Diopters means a focal length of one-half a metre.

The cornea (45 Diopters) and crystalline lens (15 Diopters) together deliver 60 Diopters, so focal length is 1/60 of a metre of about 1.6 cm.

Which suggests to me that a Diopter is cunningly defined as a unit of measurement for perfect sight in a 1.6cm depth eyeball, an eyeball with no need of correction.

A negative Diopter correction value is to decrease focusing strength.
A positive Diopter correction value is to increase focusing strength.

So John Gray and I do NOT see eye to eye. John’s eyes have lenses that focus too strongly whereas Chris’s lenses focus too weekly. The one thing we can agree on is that we both need corrective lenses.

My prescription reads +5.75 and + 4.75, which means that my eyes in their old age natural state focus behind my retina. John’s prescription at -15 means that his eyes focus in front of his retina.

We both have eye problems, and we both need corrective measures in order to see clearly.

“Unfortunately, between age 40 and 50, the crystalline lens hardens up and loses the ability to provide those extra Diopters needed when looking up close”
If I knew John’s readings for the corneal lens and for the crystalline lens, I could hazard a suggestion to the primary area of loss of focusing strength. |Since the crystalline lens deterioration is age-related, and John has cataract surgery, I’d have to bet that it was the cataracts that were contributing to his -15 readings. But I am wandering far from my area of expertise here.

I shall pump Tammy for more understanding when I go to collect my new reading glasses in a week or two.

In the meantime I suspect that chances are strong that at least one member of Eileen’s Lounge would find a + reading for one eye and a – reading for the other eye, were they to get their eyes tested.
Cheers, Chris

[HansV]

Like John Gray, I am nearsighted, although not as much.
As I get older, I become gradually less myopic - I used to have -4 / -5, and it's now down (or up) to -2.25 / -3.5.
I'll probably have to undergo cataract surgery eventually, but that's still some years away.

[ChrisGreaves]

... I used to have -4 / -5, and it's now down (or up) to -2.25 / -3.5. ...

Hi Hans.
To me this translates to "I am moving away from the John Gray orbits () and towards the Chris Greaves orbits.

Old Age, I think, means a hardening of the crystalline lens, so it follows that John, Hans, and Chris will all be affected.
I don't expect Hans to increase his prescription until it crosses the -0, -0 boundary. That is, I don't expect Hans to migrate towards perfect teenage vision. Hans has nothing to look forward to, even if he could focus

Instead our three prescriptions are based on eyeball size. That suggests, assuming that our lenses were perfectly flexible at birth, that our eyeballs are different sizes, not the average of 1.6 cm.

If in our teenage years we had perfect 0,0 vision, then we have become victims of our individual eyeball sizes.
I think then that there must be people with a + prescription for one eyeball and a - prescription for the other eyeball.

Cheers, Chris

[stuck]

...Hans has nothing to look forward to...

That's a bit bleak isn't it?

I'm sure he's at least looking forward to the next time I'm travelling and a volcano goes off...

Ken