The conversion between forms is as follows:
To convert plus cyl to minus cyl:
1. Add the cylinder power to the sphere power
2. Change the sign of the cyl from to -
3. Add 90 degrees to the axis is less than 90 or subtract 90 if the original axis is greater than 90.
To convert minus cyl to plus cyl:
1. add the cylinder power to the sphere
2. Change the sign of the cylinder to from - to
3. Add 90 to the axis if less than 90 or subtract if greater than 90
For example:
-1.25 -2.50 X 55 is the same as -3.75 2.50 X 145.
In both notations, the power in a given meridian is identical.
The power created at axis 55 is -1.25
The power created at axis 145 is -3.75
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