Prescription Analysis
Reading and Interpreting the Rx
An ophthalmic prescription lists sphere, cylinder, axis, add, and sometimes prism for OD (right), OS (left), or OU (both eyes). DS means sphere only with no cylinder.
Transposition
Transposition converts a prescription between plus-cylinder and minus-cylinder form without changing the correction. The lens power represented is identical in either form.
Prism by Decentration
Intentional or accidental decentration of the optical center from the visual axis induces prism per Prentice's rule, used to prescribe prism or avoided to prevent unwanted prism.
PD and Segment Measurement
Monocular PD is measured from the bridge center to each pupil, capturing facial asymmetry. Near PD is smaller than distance PD due to convergence. Segment height positions the reading area.
Effective Power and Neutralization
Vertex distance and lens tilt change a lens's effective power at the eye, most significantly in high prescriptions, requiring compensated power. Lensmeter neutralization finds the optical center and reads power.
Last updated: November 2026
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