How to Convert Diopters and Visual Acuity
When talking about vision, two different units of measurement are used that often cause confusion: diopters and visual acuity. Understanding the difference is fundamental for correctly interpreting an eye prescription.
Diopters: The Defect
Diopters (D) measure the corrective power needed. They are the eye's "error":
- Negative values (-1.00, -2.50): myopia -- the eye is too long
- Positive values (+1.00, +2.50): hyperopia -- the eye is too short
- Zero: emmetropia -- perfect vision without correction
Visual Acuity: The Seeing Ability
Visual acuity measures how well you see, tested with eye charts by the doctor:
- 20/20 (10/10): normal vision
- 20/40 (5/10): you see half the normal detail
- 20/200 (1/10): very reduced vision
Approximate Conversion Table
| Diopters | Visual Acuity | Classification |
|---|---|---|
| 0.00 | 20/20 | Normal vision |
| -0.50 | 20/30 | Mild myopia |
| -1.00 | 20/50 | Mild myopia |
| -2.00 | 20/100 | Mild myopia |
| -3.00 | 20/200 | Moderate myopia |
| -6.00 | 20/400+ | High myopia |
Why the Conversion Is Not Exact
The relationship between diopters and visual acuity is not linear and varies from person to person. Factors include pupil diameter, accommodation, astigmatism, and eye pathologies.
Visual Defect Classification
Myopia is classified by diopters: Mild (up to -3.00 D), Moderate (-3.00 to -6.00 D), High/pathological (beyond -6.00 D). Hyperopia follows an analogous classification with positive values.