The three elements of a glasses prescription are the sphere, cylinder and axis. The sphere corrects nearsightedness or farsightedness. The cylinder corrects the amount of astigmatism, and the axis shows where the astigmatism correction is located. A glasses prescription written in minus cylinder form will look very different from the same prescription written in plus cylinder form. However, they will both result in the same vision when they are fitted to the appropriate patient. Transposing glasses prescriptions allows one to change a minus cylinder prescription into a plus cylinder prescription or a plus cylinder prescription into a minus cylinder prescription.
Look at the cylinder of the glasses prescription that needs to be transposed. The cylinder is the middle of the three numbers in the glasses prescription. If the sign in front of the cylinder value is a minus, the glasses prescription is in minus cylinder form. If the sign in front of the cylinder value is a plus, the glasses prescription is in plus cylinder form.
Add the sphere value to the cylinder value. The result will be the new sphere value. This must be done with respect to the plus or minus sign in front of both the sphere and cylinder value. If both values have a minus sign, the new sphere value will be a larger negative number. If one value is a plus sign and the other is a minus sign, the new sphere value will be smaller than the original. The new sphere sign might be either a plus or a minus. If both values have a plus sign, the new sphere value will be a larger positive number.
Change the sign in front of the cylinder to the opposite of what it was originally. A glasses prescription that originally had a plus in front of the cylinder will now have a minus. One that originally had a minus in front of the cylinder will now have a plus.
Add 90 degrees to or subtract 90 degrees from the axis value. The axis value must fall between 0 and 180. If the original axis value is 30, 90 degrees would be added to make the new axis value 120. If the original axis value is 100, 90 degrees would be subtracted to make the new axis value 10.
Using a calculator may be helpful when adding positive and negative numbers. Calculators are useful when adding or subtracting ninety degrees to or from the axis. Double-check each transposed prescription to ensure that it is accurate.
Making a mistake while transposing a glasses prescription will result in someone receiving an incorrect glasses prescription.