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

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**Lecturer (Nethradhama School of Optometry)**

Moptom