A cylindrical lens used to correct for astigmatism may be defined either with a positive or negative cylinder. A prescription can be transformed into the other kind of lens using a number of simple rules.
Format for a cylindrical lens prescription: <spherical lens> <cylinder> x <axis>
A positive (+) spherical lens indicates a plus (convex) lens for a patient with hyperopia.
A negative (-) spherical lens indicates a minus (concave) lens used for a patient with myopia.
A positive (+) cylinder indicates a plus (convex) cylinder for a patient with astigmatism.
A negative (-) cylinder indicates a minus (concave) cylinder for a patient with astigmatism.
Transformation for the prescription S1 C1 x A1
New prescription S2 C2 x A2
S2 = S1 + C1
C2 = (-1) * (C1)
A2 = either (A1 + 90 degrees) or (A1 - 90 degrees), whichever gives a positive number less than 180°
Converted Parameter
New Values
sphere
(original sphere) + (original cylinder)
cylinder
(-1) * (original cylinder)
degrees
(degrees) + 90 (if positive number and < 180) OR
(degrees) - 90 (if positive number and < 180)
where:
• An original degrees of 90 could convert to 0 or 180.
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