Sphreq.txt
SUBJECTS: 1. Spherical equivalent conversion
2. Two astigmatic "prescriptions".
We often get a "astigmatic" prescription which looks
terrible. Here is how to convert this prescription into a
spherical equivalent.
Further, we can get "astigmatic" prescriptions from an MD and
an OD, and they appear to be profoundly different. Often they are
not. There are small variations between them that "confound" your
judgmentent about a comparision. Under this circumstance, convert
them to "spherical equivalent" to get a realistic comparision.
Best,
Otis
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1. Spherical equivalent conversion
From: "Mike Tyner"
Spherical equivalent is the mean dioptric power, the average
of the two major meridians.
For -0.75 +0.25 x 040,
the Spherical Equivalent (SE) is -0.62.
When you convert, honor the signs.
Your myopia would have to double before it threatens your
qualification. And the later myopia begins, the milder it tends
to be.
-MT
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2. Two astigmatic versions
Re: Just today I got examined by an Optometrist who gave me the
following prescription: How do I make these conversions?
(Right) OD +.75 -1.00 115 the newer right converts to
OD -.25 +1.00 X 25
(Left) OS -0.25 -1.75 88 the newer left converts to
OS -2.00 +1.75 X 178
Prescription from my Ophthalmologist 3 months ago was:
R Plano
L -2.25 +1.50 X 180
I had macular hole surgery on my right eye in June. It
appears that I have developed astigmatism in the Right Eye.
However the left eye does not appear to have changed much. Are
there two ways of writing eye prescriptions and if so how do they
compare? thanks Bill
_______________________
It is the same only written differently. Minus cylinder and
plus cylinder powers.
the newer right converts to -.25 +1.00 X 25
the newer left converts to -2.00 +1.75 X 178
The formula is:
add the sphere power to the cylinder power (1st and 2nd
numbers) for a new sphere total, reverse the sign of the cylinder
power, add or subtract 90 (so the new number is less than or equal
to 180) for the new axis.
To understand that they are the same, think of this:
Using the new left eye powers:
"The power is -2.00 at axis 178 and the power is (the sum of
the two numbers) -.25 at axis 88 (perpendicular, 90 degrees
away)."
As it was written the first time,
"The power is -.25 at axis 88 and -2.00 at axis 178.
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Here are some remarks on primate astigmatism for
your interest.
From: The Distribution of Refractive Errors in Monkeys", by
Francis Young, 1964
Subject: The percentage of mild astigmatism in prmates.
The minimum measurement for astigmatism is 1/4 diopters. It
is then judged in 1/4 dioter steps. Mild is up to 1 dipoter.
Here are the remarks:
"Approximately 9.5 % of the monkey eyes showed some degree of
astigmatism. (of Francis Young's measurements. OSB)
This value may be compared with 10.5 % found by Jablonsky
(1926) in 57 primate eyes. Four of his six astigmatic eyes were
found in three baboons.