What it means and why
Spherical equivalent (SE) summarises sphere and cylinder in one number: SPH + CYL ÷ 2, keeping the cylinder sign. In a simple spherocylindrical prescription it is the midpoint of the two principal powers.
That compression deliberately loses information. SE does not retain the cylinder amount or axis, so two prescriptions with identical SE need not provide the same correction. It is useful for describing or comparing certain measurements, not as a full replacement for an order.
A worked example
Teaching example: −2.00 SPH with −1.00 CYL gives SE −2.50 D. A second record of −2.50 SPH with no cylinder has the same SE, yet lacks the directional difference of the first. Averaging did not make the two records interchangeable.
What it does not tell you
SE alone does not measure axial length or eye health. Research thresholds using SE apply with stated measurement conditions, including whether accommodation was controlled.
THINK IT THROUGHWhat disappears when you reduce a prescription to SE?
The separate cylinder magnitude and its axis. Keep the original prescription alongside any summary.
Sources & reading notes
Page updated:
- International Myopia Institute · Defining and classifying myopia ↗ Research guidance
Check the report’s definitions and intended population before applying a conclusion.