How to understand it
Useful in some classifications and research, it discards cylinder orientation and cannot reproduce the full prescription.
An example
Teaching example: −2.00 + (−1.00 ÷ 2) = −2.50 D. The original cylinder and axis still matter.
A common mix-up
Do not order spectacles or contacts by substituting SE for the prescribed fields.
See the idea in a full guide →
Sources & verification
Page updated: