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Part 04 - A Prior Prescription Exists
The choice between concave (negative) and convex (positive) cylinder lenses can affect the accuracy of refraction. For example, a prescription written as +1.00 sphere with -1.00 cylinder at 180° partially cancels itself out-making testing less precise.
This prescription is mathematically equivalent to 0.00 sphere with +1.00 cylinder at 90°. In such cases, using convex cylinders simplifies testing and often improves clarity.
The calculations can be done manually:
Manual Polarity Test
Step 1: Add the left and right sphere values together using signed arithmetic.
Step 2: Multiply that result by 2.
Step 3: Then add both the left and right cylinder values.
If the final sum > 0, use positive (convex) cylinders.
If the final sum < 0, use negative (concave) cylinders.
Reversing Prescription Polarity
Add the sphere and cylinder values together (signed arithmetic) to determine the new sphere.
Reverse the sign of the cylinder.
Add 90° to the axis. If the new axis exceeds 180°, subtract 180° to bring it back into the 0-180 range.
Proceed to Part 05 - A Prior Prescription Exists, Test One Eye