Depending on the equipment used by the doctor, a prescription may be in plus or minus cylinder form. For surfacing, the lab uses the minus form, meaning if a prescription is written in a plus cylinder form it must be transposed before processing.
Transpose a prescription written in plus cylinder form to minus cylinder form as follows:
1. Add the sphere and cylinder powers to determine the new sphere power.
2. Change the sign of the cylinder.
3. Change the axis by 90 degrees.
Example:
Transpose -3.00 +2.00 x 30
1. Add the sphere and cylinder powers to determine the new sphere power.
(-3.00) + (+2.00) = -1.00
2. Change the sign of the cylinder
-2.00
3. Change the axis by 90 degrees.
120
The transposed prescription is:
-1.00 -2.00 x 120
