Myopia and reading glasses

I have been myopic (around 4.5 diopters correction) for a long time.
Recently I began to suffer from presbyopia which I understand is
very common at my age (early 40's). I have a Phd in physics and
spend a great deal of time on a computer or doing close work of
one sort or another so I elected to get a pair of glasses
specifically for that purpose. These are not bifocals or progressives
just my current prescription for myopia but 1 diopter weaker. It
was a great relief being able to read clearly again but my distance
vision was blurred as expected -- roughly 20/50 at first with the
new glasses so I would wear my standard prescription when I was not
engaged in close work. After a few weeks I seemed to be seeing
better at distance with the new pair and I gradually stopped
wearing the older pair even for distance work. My distance vision
measured with the new glasses has gradually improved and I can
now read the 20/20 line on the eye chart.

It's nice not having to deal with bifocals but I am very curious
as to what caused the improvement in my distance vision. Because
of my background I am skeptical of the explanations offered by
advocates of natural vision correction techniques but I don't
see anything in the generally accepted explanations for myopia
that would explain this. (Sure if I was younger or my original
prescription was too strong but I'm in my 40's and had the
same correction for years).

One thing that occurred to me when I started reading about
the human visual system was that the way a correction is defined
is quite vague. A single diopter value for instance can only be
correct for a single wavelength of light because the index of
refraction of the eye's focusing sytem varies with wavelength.
I have not worked out the numbers but I would not be surprised
if there was a couple of diopters worth of difference over the range
of visible light. So what does the number actually represent?
The value at some specified wavelength perhaps? How does the
eye compensate for these differences?

Emil

 

The simple explanation, and most likely, is that your previous -4.5 Rx
was wrong, and overcorrected by something close to one diopter. You were
not actually truely prebyopic but experiencing the symptoms of such
because your Rx required focusing an extra diopter, which was a diopter
more than you very able to do comfortably.

And perhaps is was wrong for years, or your myopia improved somewhat and
the decreased power was not incorporated in updated prescriptions, or
your Rx was generated when you were over-focusing, or your crystalline
lens flattened, or shifted posteriorly, or cornea flattened----as you
can see, there a a number of possible explanations.

It doesn't, as it functions, and is measured, based on the average of
all visible wavelengths, or at least the median of it, or in the testing
situation, something close to "natural" light. The difference in testing
over the wavelength range of "regular" incandescent or fluorescent
lighting is less than .1 diopter. And if I remember my phyisical optics
(and I'm sure one can look this up), the difference between the
refractive index from the longest red to the shortest blue-violet is
around .2D and the difference in the index of refraction is about .02.

The info is published somewhere, I'm certain, but you might enjoy doing
the math yourself, using Snell's Law. Here's a website that gives the
basic info.
http://www.rdoman.com/phy/rrp/rrp51/

--LB

--
Dr. Larry Bickford, O.D.
Family Practice Eye Health & Vision Care

The Eyecare Connection
http//www.eyecarecontacts.com
larrydoc at eye-care-contacts dot com (remove -)

 

One thing that occurred to me when I started reading about

Thanks but I had heard of Snell's law before (it's hard to get
a Phd. in physics otherwise). The refractive index is a
dimensionless quantity so I am not sure what you mean by saying
that the difference in refractive index from longest red to
shortest blue is .2D -- one doesn't measure refractive index
in diopters.

In any case one of the the missing pieces of information needed to
compute the shift from red to blue wavelengths is the dispersion
relation for the human eye.Poking around online I found this reference

http://www.bradford.ac.uk/acad/lifesci/optometry/resources/modules/stage1/pvp1/ChromAb.html

I don't know how accurate this source is. It does suggest that
the longitudinal chromatic aberration is more on the order of
2D instead of 0.2. That's what I would have expected from the
physical dimensions of the human visual system (assuming a
dispersion relation similair to that of glass). You can get
a rough estimate of that from the lensmakers equation

1/f = (n-1)*(1/r1 - 1/r2)

where f is the focal length of the lens system, n is the
refractive index at a specific wavelength and r1 and r2
are the radii of curvature of the two sides of the lenses.
For fixed R1 and R2 we then find that the percentage change
in the power of the lens D=(1/f) is approximately the same
as the percentage change in the index of refraction. Since
the human eye has a power of a couple of hundred D that
means the shift from red to blue assuming a 1% change in
the index of refraction is around 1 to 2 D.

The reference given above offers some explanations as to
why the chromatic aberration does not have a greater impact
on our visual acuity -- but once again I don't know how to
evaluate the accuracy of that reference.

Regards
Emil

 

Your RX was probably too strong in the first place.

A dioptre is defined as a lens that brings to focus a parallel ray of
light at 1 meter.

Yes, that is the basis of the duochrome (red-green) test. The test
takes advantage of the eyes chromatic abberation. When a myopic eye is
undercorrected red light will reach the retina first, and is therefore
more focused then the green light. If over corrected, the green light
is more in focus. If I remember correctly, the mid point between these
two color frequencies would theoretically be that of sodium yellow
light. The index of refraction of the lens material would be
calculated using sodium yellow light.

 

I was also skeptical of natural vision correction techniques until I tried them. The reason I did was that my vision problems had forced me to limit the scope of my consulting engineering practice by not allowing me to work more than 2-3 hrs. per day on the computer. In short, my problem depended on the constant eye strain, caused by myopia, astigmatism, and an eye imbalance. As the result I had constantly inflamed eyes, which did not respond to any type of eye drops or drugs.
After unsuccessful attempts by various optometrists and ophtamologists to alleviate my problems, and after trying multiple single, bi, and multifocal glasses, I decided to conduct my own research. In 1997, I attended a short natural vision seminar. At the seminar, although very skeptical, I experienced an immediate eye strain relief after doing a few simple exercises.
Still in a mood of disbelief, I decided to give the eye exercises a try, on the premise that at worst they wouldn't hurt. After two months of doing them, I noticed a definite improvement in my vision, allowing me to work long on the computer, and without glasses. I also started having flashes of perfectly clear far vision without glasses. At that point I decided to get serious about that. I read almost all available materials and books on the natural vision improvement and found that some of them explained in a very logical manner the phenomena that had caused my problems.
After doing vision exercises for six months, I saw my optometrist, who was stunned to witness the reduction of my myopia from -3.5 (sphere & cyl. combined) to -2.25. He also prescribed me, at my request, 1 diopter weaker glasses (-1.25). Those were the glasses I used on and off for the next year (mostly for driving). After that, I stopped using glasses at all (after 23 years of wearing them) and my major eye problems gradually disappeared.
Since then, I have continued to further exercise my eyes in order to eliminate presbyopia, which is typical for people after 40 (I'm in my 50s). Presently I can see well both at close and far distance without glasses. Actually, I enjoy reading very small print (size 2.5), which improves my focusing abilities.

My success in this respect does not mean that it has been easy. I have put A LOT of effort and time in making my eyes work, and I still have to keep working on my vision habits so that I don't have the recurrence of my problems. That is probably why the mainstream medicine is not accepting the natural vision improvement as a viable method - it requires a long and sustained commitment and effort from the patient.

Aldus Huxley, the renowned western philosopher, avoided an imminent blindness by applying the natural vision improvement method. As a tribute to its originator, Dr. Bates, he wrote the book "The Art of Seeing" (ISBN 0-00-654746-X).

--
John Schindler, M.Sc., P.Eng.

SCHINDLER TECHNOLOGIES CORPORATION
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