Optometrists and lens makers measure lens strength in diopters: the reciprocal of the focal length in metres. A +2 D reading lens has a focal length of 50 cm; a −4 D lens for short-sightedness diverges light as if from 25 cm.
How to use the Diopter Calculator
- Focal length f
- Every result field is editable: type into Optical power P and Focal length f plus every other field update to match.
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the focal length, or type a power into the result to find the focal length. Negative values describe diverging lenses.
Example values
How the main result changes when Focal length f changes and everything else stays at the example values above:
| Focal length f (cm) | Optical power P (D) |
|---|---|
| 25 | 4 |
| 37.5 | 2.6667 |
| 50 | 2 |
| 62.5 | 1.6 |
| 75 | 1.3333 |
| 100 | 1 |
Formula
P (D) = 1 / f (m)
Worked example
Inputs — Focal length f: 50 cm.
Results — Optical power P = 2 D.
Key terms
- Diopter
- Unit of optical power, 1/m.
Frequently asked questions
What is a diopter?
The reciprocal of focal length in metres (m⁻¹).
What does a negative diopter mean?
A diverging (minus) lens, used for myopia.
How do powers combine?
Thin lenses in contact add their powers.
What is the power of the human eye?
About 60 D.