How to calculate horizontal decentration

Example in millimetres

A = 52 and bridge = 18 give 70 mm between centres. Right PD = 31 and left PD = 33 give 4 mm and 2 mm nasal shifts.

Convention

Positive means nasal displacement from the geometric centre of each eyewire; negative means temporal displacement. Zero means horizontal coincidence.

Model limits

This assumes a symmetric frame and a central nasal reference. It does not calculate fitting height, thickness, minimum diameter, prescribed prism or design compensation. Verify measurements and conventions with the laboratory.

References

• Laramy-K / OpticianWorks — ABO Study Guide

Related topics

• Frame geometry: A, B and bridge
• Pupillary distance: which measurements should you record?
• Prentice’s rule: approximate prism magnitude