Plus / minus cylinder transposition
Flip a prescription between plus and minus cylinder.
About this calculator
Every sphero-cylinder can be written in plus or in minus cylinder form, and both describe the same lens. Prescriptions arrive in either form, while contact lenses and most laboratory orders use minus cylinder.
Transposition takes three steps. Add the cylinder to the sphere to get the new sphere, change the sign of the cylinder, and turn the axis by 90°.
Formula
S′ = S + C, C′ = −C, axis′ = axis ± 90
Worked example
−2.00 +1.50 × 090 becomes −0.50 −1.50 × 180.
Sources
- System for Ophthalmic Dispensing, ch. 12, Toric Transposition