What it means and why
CYL describes a difference in prescribed power between two principal directions. It needs AXIS to specify orientation. Think of amount and direction as a pair: recording only the amount leaves the correction incompletely described.
Prescriptions may be written in plus-cylinder or minus-cylinder notation. Equivalent records can therefore look different in SPH, CYL and AXIS at the same time. A sign change in CYL alone is not enough to infer worsening or improvement.
A worked example
Teaching example: SPH −2.00 D, CYL −1.00 D at 180° describes principal powers of −2.00 D and −3.00 D in perpendicular directions. Adding absolute values and calling the result a universal “−3.00 lens” loses that directional difference.
What it does not tell you
The cylinder number is not a separate spherical prescription and should not be added to every viewing direction. Compare full records in the same notation with the prescriber.
THINK IT THROUGHIs CYL −1.00 D enough to make an astigmatic lens?
No. The axis, spherical component and eye assignment are also needed, along with dispensing measurements.
Sources & reading notes
Page updated:
- AOP · Your prescription explained ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.
- ABDO · Explaining spectacle prescriptions, part 1 ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.