Before revision: There is no difference in these forms of notation and it is easy to convert between them: Add the sphere and cylinder numbers together to produce the converted sphere Invert the sign of cylinder value Add 90° to axis value, and if the new axis value exceeds 180°, subtract 180° from the result For example, a lens with a vertical power of -3.75 and a horizontal power of -2.25 could be specified as either -2.25 -1.50 x 180 or -3.75 +1.50 x 090.
After revision: There is no difference in these forms of notation and it is easy to convert between them: Add the sphere and cylinder numbers together to produce the converted sphere Invert the sign of cylinder value Add 90° to axis value, and if the new axis value exceeds 180°, subtract 180° from the result For example, a lens with a vertical power of −3.75 and a horizontal power of −2.25 could be specified as either −2.25 −1.50 × 180 or −3.75 +1.50 × 090.