Transposing a Rx prescription is simply converting the prescription from minus cylinder notation to plus cylinder notation. The optical properties of the prescription remain the same.

Procedure:
1) Add the cylinder power to the sphere power to arrive at the new sphere power.

2) Change the sign of the cylinder power.

3) Add or subtract 90 from the axis.

Example 1— Transpose the following prescription:

+2.00 – 2.50 x 105

1) +2.00 – 2.50 = – .50 (new sphere)

2) -2.50 changes to +2.50 (new cyl.)

3) 105 – 90 = 15 (new axis) * your axis transposition can never be greater than 180

Answer: -.50 +2.50 x 15

Example 2 —

+1.00 + 3.00 x 35

1) +1.00 + 3.00 = +4.00

2) +3.00 changes to – 3.00

3) 35 + 90 = 125

Answer: +4.00 – 3.00 x 125

Why are there plus and minus cylinders? Plus cylinder lenses exist only in phoropters and trial lenses. Plus cylinder phoropters are popular because it is easier to teach retinoscopy in plus cylinder. Ophthalmologists typically learn retinoscopy and refraction in plus cylinder. Optometrists typically use minus cylinder.