Transposition: another notation for the same prescription

Example

−2.00 −1.00 × 180° is equivalent to −3.00 +1.00 × 90°. The operation changes notation; it does not determine a new prescription.

Units and axis

Sphere and cylinder are in dioptres; axis is in degrees. The calculator accepts axes from 0° to 180° and reports 1° through 180° for integer axes. With zero cylinder, axis has no optical effect and is preserved, normalizing 0° to 180°.

Verification

Reverse transposition should recover the original representation, accounting for the equivalence of 0° and 180°. It does not fill missing fields or calculate addition or prism.

References

• Ohio Optometric Association — Optometric Math

Related topics

• Prentice’s rule: approximate prism magnitude
• Lens thickness: inputs needed for an estimate