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 +++++++++++++++++++++++++++++++++++ 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 +++++++++++++++++++++++++++++ 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. ================= 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.