Focal length of a extremely myopic eye?

What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
that the range between my near point and far point?

In article
<83873c04-c7a7-41bf-ad10-(E-Mail Removed)>,
douglas <(E-Mail Removed)> wrote:

> What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> that the range between my near point and far point?

1. Don't know.
2. No. The range between your near point and far point varies. Those
numbers have nothing to do with it.

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

Dear Bill,

The standard power of the "normal" eye is given as about 60 diopters.

Enjoy,



On Aug 17, 8:53*pm, Salmon Egg <Salmon...@sbcglobal.net> wrote:
> In article
> <83873c04-c7a7-41bf-ad10-0773063ff...@18g2000yqa.googlegroups.com>,
>
> *douglas <protoman2...@gmail.com> wrote:
> > What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> > that the range between my near point and far point?
>
> I have seen optical models of the eye, in the OSA Handbook, for example.
> From memory, the eye is about 25mm diameter making its lens power 40D.
> add your prescription algebraically to to get 28.75D. Take the
> reciprocal to obtain a focal length of about 35mm. That is an indication
> of how long your eyeball is. It is possible to calculate more accurately
> with the model, what is the point?
>
> Bill
>
> --
> Private Profit; Public Poop! Avoid collateral windfall!

Dear Bill,

If memory serves me correctly, the "standard power" of the eye is
given as about 60 diopters.

Enjoy,



On Aug 17, 8:53*pm, Salmon Egg <Salmon...@sbcglobal.net> wrote:
> In article
> <83873c04-c7a7-41bf-ad10-0773063ff...@18g2000yqa.googlegroups.com>,
>
> *douglas <protoman2...@gmail.com> wrote:
> > What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> > that the range between my near point and far point?
>
> I have seen optical models of the eye, in the OSA Handbook, for example.
> From memory, the eye is about 25mm diameter making its lens power 40D.
> add your prescription algebraically to to get 28.75D. Take the
> reciprocal to obtain a focal length of about 35mm. That is an indication
> of how long your eyeball is. It is possible to calculate more accurately
> with the model, what is the point?
>
> Bill
>
> --
> Private Profit; Public Poop! Avoid collateral windfall!

On Aug 17, 5:01*pm, douglas <protoman2...@gmail.com> wrote:
> What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> that the range between my near point and far point?

Not sure what you mean by "focal length of my eye". Do you mean the
axial length? You can't calculate your axial length from your
prescription, you need to have it measured.

Judy

On Aug 17, 8:22*pm, Dr Judy <mpac...@rogers.com> wrote:
> On Aug 17, 5:01*pm, douglas <protoman2...@gmail.com> wrote:
>
> > What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> > that the range between my near point and far point?
>
> Not sure what you mean by "focal length of my eye". *Do you mean the
> axial length? *You can't calculate your axial length from your
> prescription, you need to have it measured.
>
> Judy

Focal length, like on a camera. Would a severely myopic eye be like a
telephoto or a wide-angle, fast or slow lens? What exactly does the
value in mm mean? The longest distance before an image goes out of
focus?

In article
<174d4430-2a5d-4ec4-951b-(E-Mail Removed)>,
Dr Judy <(E-Mail Removed)> wrote:

> On Aug 17, 5:01*pm, douglas <protoman2...@gmail.com> wrote:
> > What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> > that the range between my near point and far point?
>
> Not sure what you mean by "focal length of my eye".

Focal length (FL) is a pretty common optical term. It should be
measurable. I suspect it's pretty useless, though. Most people, unlike
me, have accommodation. You are the expert here, and perhaps there are
other factors. What difference does it make? If it gets measured at
one point in time, and you look at something at a different distance, it
won't be the same.

> Do you mean the
> axial length? You can't calculate your axial length from your
> prescription, you need to have it measured.

I had mine measured. I don't know what it is. The surgeon needed to
know, for my cataract surgery.

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

On Aug 17, 11:04*pm, Dan Abel <da...@sonic.net> wrote:
> In article
> <174d4430-2a5d-4ec4-951b-f839646c0...@h21g2000yqa.googlegroups.com>,
> *Dr Judy <mpac...@rogers.com> wrote:
>
> > On Aug 17, 5:01*pm, douglas <protoman2...@gmail.com> wrote:
> > > What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> > > that the range between my near point and far point?
>
> > Not sure what you mean by "focal length of my eye".
>
> Focal length (FL) is a pretty common optical term. *It should be
> measurable. *I suspect it's pretty useless, though. *Most people, unlike
> me, have accommodation. *You are the expert here, and perhaps there are
> other factors. *What difference does it make? *If it gets measured at
> one point in time, and you look at something at a different distance, it
> won't be the same.
>
> > *Do you mean the
> > axial length? *You can't calculate your axial length from your
> > prescription, you need to have it measured.
>
> I had mine measured. *I don't know what it is. *The surgeon needed to
> know, for my cataract surgery.
>
> --
> Dan Abel
> Petaluma, California USA
> da...@sonic.net

Focal length is the reciprocal of the lens strength. Minus lens make
it shorter, plus lenses make it longer. It moves the near/far point of
your eyes. I think

Douglas,

There are two "lengths" to consider.

In the normal adult eye, the power is given as 60 diopters (in some
texts, 58 diopters.)

The average phycial length is about 2.4 cm.

The OPTICAL length is about 1.7 cm.

You can calculate both these dimensions as a difference from 58
diopters.

It took about a -12 dioper lens to "Clear your Snellen", so your total
power is calculated as:

58 diopters + 12 diopters = 70 diopters.

Your requested dimensions can be calculted from 70 diopters. (See my
book for these equations.)

The "range of accommodation" depends on age, and for the young can be
as much as 12 diopters (great variation).

As you get older that "range" decreases, so that at age 50 it may be
as little as 1 to 2 diopters.

You could also determine this by objective testing.

Enjoy,









On Aug 17, 5:01*pm, douglas <protoman2...@gmail.com> wrote:
> What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> that the range between my near point and far point?

Douglas,

With the information you provided, you can not calculate the "length"
of your eye -- if that is what you are asking for.

But the normal adult eye has a length (phycial) of 2.4 cm, and a power
of 58 diopters. (Standard value, provided by Gullistrand.)

With your "nearsighted" eye, you would be able to see clearly at 40
inches / 12 = 3.3 inches.

Move the object away to the distance, and the image will form in front
of the retina.

This means that, with the value you provided (12 diopters spherical
equivalent), your focal length is:

Power = Index-of-Refraction / Distance

Distance = Index / Power

Distance = 1.38 / 70

0.02 meters (approximate)

Thus, your eyes with a power of 70 diopters will form a image 0.024 -
0.020 = 0.004 meters in front of your retina.

It is important to understand that the optical length is different
than the physical length. This is because the speed of light is
slower in water than in air.

The length that MIke refers to (17 mm) is the length in a medium with
a refractive index of 1.38.

Enjoy,

=========


On Aug 17, 5:01*pm, douglas <protoman2...@gmail.com> wrote:
> What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
> that the range between my near point and far point?