It’s a method of converting spectacle prescriptions from one cylinder form to another i.e. plus cylinder to minus cylinder or vice-versa
Steps of simple transposition:
Step 1: Axis change:
If the axis of the cylinder is equal or less than 90 degrees, add 90 degrees
If the axis of the cylinder is more than 90 degrees, subtract 90 degrees
Step 2: Cylindrical changes
The cylindrical power always remains the same but the sign of cylindrical power always will be the opposite.
Step 3: Sphere changes
Add spherical and cylindrical power to spherical power
Example 1
+1.00DS / +1.00 DC X 90
Step 1:
Add 90 to axis of the cylinder to find new cylinder axis
New cylinder axis = Axis of old cylindrical power + 90 (because axis of cylinder is equal or less than 90 degrees)
New cylinder axis = 180
Step 2
The cylindrical power always remains the same but the sign of cylindrical power always will be the opposite.
New cylindrical power = -1.00 DC
Step 3
Add old spherical and old cylindrical power to get new spherical power.
New spherical power = Old spherical power + old cylindrical power
New spherical power = +1.00 + (+1.00)
New spherical power = +2.00 DS
Lens Prescription form = New spherical power (Step 3) / New cylindrical power (Step 2) X Axis of the new cylinder (Step 1)
Lens Prescription form = +2.00 DS / -1.00 DC x 180
Example 2:
+2.00DS / -2.00 DC X 100
Step 1:
Subtract 90 to the axis of the cylinder to find the new cylinder axis
New cylinder axis = Axis of old cylindrical power – 90 (because the axis of the cylinder is more than 90 degrees)
New cylinder axis = 100 – 90 = 10 degrees
Step 2
The cylindrical power always remains the same but the sign of cylindrical power always will be the opposite.
New cylindrical power = +2.00 DC
Step 3
Add old spherical and old cylindrical power to get new spherical power.
New spherical power = Old spherical power + old cylindrical power
New spherical power = +2.00 + (-2.00)
New spherical power = Plano
Lens Prescription form = New spherical power (Step 3) / New cylindrical power (Step 2) X Axis of the new cylinder (Step 1)
Lens Prescription form = Plano / +2.00 DC x 10
For more information, Please visit our YouTube channel.
Lecturer (Nethradhama School of Optometry)
Moptom