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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.

OD = right eye, OS = left eye, OU = both eyes.

The value after 'x' is the cylinder axis (1 to 180 degrees).

Prism notation: BO base out, BI base in, BU base up, BD base down.

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.

Add sphere and cylinder to get the new sphere.

Change the sign of the cylinder.

Rotate the axis by 90 degrees.

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.

Prism (prism diopters) = decentration (cm) x power (D).

Decentration (cm) = prism / power.

Raising the OC of a plus lens creates base-down prism; lowering it creates base-up.

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.

Monocular PD accounts for facial asymmetry better than a split binocular PD.

Near PD is about 3 to 4 mm less than distance PD total.

Seg height is measured vertically from the lowest lens edge to the top of the segment.

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.

Spherical equivalent = sphere + one-half the cylinder.

Moving a minus lens farther from the eye increases its effective minus power.

Neutralization centers the lensmeter mires to find the OC and read sphere, cylinder, axis, and add.

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Last updated: November 2026

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