How to understand it
It describes a difference between principal directions and needs an axis. Prescriptions can use plus- or minus-cylinder notation.
An example
CYL −1.00 D without AXIS leaves the orientation unspecified, so it is not a complete instruction for that correction.
A common mix-up
Do not add the absolute SPH and CYL values to make one “total power”.
See the idea in a full guide →
Sources & verification
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