Ok, let's look at the eye itself, so the eye,
it turns out, is not entirely,
I mean, it is global, but it's not entirely
So the transverse diameter is there and
there is some variability.
You can't have people who have little eyes
or people who have larger eyes.
But somewhere in that 21 to 27 millimeters
is kind of the lateral dimension.
The anterior-posterior dimension is actually
a little bit more squished.
So this is why it's not completely spherical
And so the actual dimension is a little bit
less than the transverse dimension, always
OK. The important point about all the
structures of the eye is actually to focus
light appropriately on the retina.
That's what we're trying to do.
So there is a certain focal length that
extends from the lens into
the retina, and that's that green triangle
there in the middle.
And when everything is perfect, we have
bringing the light in is a combination.
Focusing it appropriately is a combination
not only of the lens, but also of the
cornea. And so that's going to be an
It turns out that some of the water, some of
the tear fluid and other fluids
released by conjunctiva are also going to be
important for kind of the important
refract tile focusing of light.
But predominantly it's going to be lens and
the cornea and it
focuses light appropriately on the retina.
Now, it doesn't always work perfectly.
This is an exaggeration.
It's not ever quite this bad.
But this is an ametropia as opposed to
or refractive error.
And it means that the light entering the eye
does not focus appropriately on the
retina. So what the retina sees will be
This can be a combination of effects related
to the cornea and the
corneal curvature or the lens.
And it really the ultimate result is that we
don't have the
capacity to focus the light so that it's a
nice pinpoint thing
back on the retina and you can see where its
It's kind of in the middle of the posterior
segment of the eye.
So Amaterasu is divided into two kind of
nearsightedness, or myopia, or
farsightedness, hyperopia. So we're going to
talk about myopia first and then we'll talk
What happens in myopia is that the lens is
curved or the cornea is too curved or the
actual length of the eye is too
long. And as a result, light coming in is
focus back on the retina, which is that
yellow structure, and that will make you
nearsighted. This is frequently so the
actual length of the eye being too
long is a frequent reason why young adults
they're growing may develop nearsightedness
that the eye
actually orbit grows a little bit too
quickly relative to the rest of
the structures that may catch up over time.
Also, as the lens curvature changes with
And we'll talk about that.
OK, so that's what happens with
So how do we fix that?
The light is focused in front of the retina.
We can only focus on objects that are near,
Objects tend to be out of focus.
So what we do is we call it we put a lens in
front of the cornea
and that is glasses as such as you see on
And we cause the light to diverge ever so
So we have concave lenses that caused light
And now when it hits the cornea in the lens,
the exact appropriate refraction to have it
on the retina.
So thank goodness for that, because
otherwise I'd be looking at you through, I
could, you could only look fuzzy to me.
Okay, hyperopia, they kind of flip side of
the coin, farsightedness, the
focal length is too short.
So the retina is too short relative to where
the lens and the cornea are focusing the
light. This can also happen because the
cornea is too flat or the lens
is too flat. And as a result we focus
beyond where the retina is not good
Absolutely. So we can only usually focus on
things that are far away, near objects will
tend to be out of focus.
How do we fix that? Well, we use convex
lenses that cause
convergence of the light, so we allow it to
focus now appropriately
on the retina. Pretty simple, which keeps
keeps optometrist's in business.
Astigmatism is a little bit more complex,
but kind of the same process if there is a
symmetry of the eye so you can see that the
eye is really not round
or there's uneven curvature of the lens or
of the cornea.
All these things will cause light to come in
and be refracted in ways that are not clean
and pure. And so you have uneven focus in
much whether you're looking at things far
away or things that are near.
So astigmatism is just asymmetry of the
the eye, asymmetry the lens, asymmetry of the
cornea you have
end up with multiple focal points and things
look fuzzy everywhere.
Is that the end of the game?
Can we fix this? Well, absolutely, because
what we can do is we can put in an
appropriate corrective lens that's
asymmetric and allows the light
to be focused, takes a little bit more
creativity, a little bit more work on the
part of the optometrist.
But fortunately, because most of us don't
have a purely
spherical eye, this is something that can be
done for everybody.