By Zencalcy Editorial TeamLast reviewed About us
Optometrists describe a lens by its power in dioptres rather than its focal length. One dioptre is the power of a lens with a one-metre focal length, so stronger lenses have shorter focal lengths and higher dioptre values.
Optical power in dioptres from focal length.
P = 1 / f
P is optical power in dioptres (m⁻¹) and f is focal length in metres. Prescriptions add powers for lenses in contact, such as a spherical correction plus a reading addition.
| Symbol | Meaning |
|---|---|
| P | Optical power in dioptres. |
| f | Focal length in metres; negative for diverging lenses. |
P = 1 / 0.5 = +2.00 D. A diverging lens with f = −0.25 m has P = −4.00 D, the sort of value used to correct short-sightedness.
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One reciprocal metre. A +3 D lens focuses parallel light 0.333 m from its centre.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Content updated: September 2026