It is very easy to transpose glasses or spectacle prescriptions. You need to follow some basic rules to transpose minus cylinder to plus cylinder or plus cylinder to minus cylinder form.

Rule 1: To add spherical power and cylindrical power and you will get new spherical power.

Rule 2: Change the sign of the cylinder (minus to plus or plus to minus) without changing cylindrical power value.

Rule 3: Change the cylinder axis 90 degrees apart ( if you have axis 10 to 90 then add 90 and 100 to 180 then subtract 90). You will get a new axis after adding or subtracting.

**Example 1:**Sph +2.50 D / Cyl -1.50 D Axis 170

Adding Sph +2.50 D and Cyl-1.50 D = New Sph +1.00 D

So, here +1.00 D is new spherical power.

Changing the sign of cylinder minus to plus. So, new cylindrical power would be +1.50.

Lastly changing the axis 90 degrees apart. So, in this example axis is more than 90, you need to subtract 90 from 170. So, new axis would be 80.

**After Transposition:**

Sph +1.00 D / Cyl +1.50 D Axis 80.

**Example 2:**Sph -1.75 D / Cyl -1.25 D Axis 60

Adding Sph -1.75 D and Cyl -1.25 D = New Sph -3.00 D.

So, here -3.00 D is new spherical power.

Changing the sign of cylinder minus to plus. So, new cylindrical power would be +1.25.

Lastly
changing the axis 90 degrees apart. So, in this example axis is less
than 90, you need to add 90 to 60. So, new axis would be 150.

**After Transposition:**

Sph -3.00 D / Cyl +1.25 D Axis 150.

**Example 3:**Sph +1.75 D / Cyl -3.25 D Axis 40

Adding Sph +1.75 D and Cyl -3.25 D = New Sph -1.50 D.

So, here -1.50 D is new spherical power.

Changing the sign of cylinder minus to plus. So, new cylindrical power would be +3.25.

Lastly
changing the axis 90 degrees apart. So, in this example axis is less
than 90, you need to add 90 to 40. So, new axis would be 130.

**After Transposition:**

Sph -1.50 D / Cyl +3.25 D Axis 130.

**Example 4:**Sph +0.75 D / Cyl +1.25 D Axis 110

Adding Sph +0.75 D and Cyl +1.25 D = New Sph +2.00 D.

So, here +2.00 D is new spherical power.

Changing the sign of cylinder plus to minus. So, new cylindrical power would be -1.25.

Lastly
changing the axis 90 degrees apart. So, in this example axis is more
than 90, you need to subtract 90 from 110. So, new axis would be 20.

**After Transposition:**

Sph +2.00 D / Cyl -1.25 D Axis 20.

You should know about Refractive diagnosis also.

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