Physics

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Diopter Calculator.

Calculate optical power from focal length in metres.

On-device calculationNo signup
01

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Optical power: 2 D

Optical power

2.000000D

Results update automatically as you type.

Use Cases

Optics and Lens Design

Quickly determine the optical power of a lens or mirror from its focal length, useful for designing optical systems or verifying specifications.

Example: A lens with a focal length of 0.5 m has an optical power of 2 diopters.

Physics Education

Help students understand the relationship between focal length and optical power in physics classes, reinforcing the inverse relationship.

Example: Calculate the power of a concave mirror with a focal length of -0.25 m.

Frequently Asked Questions

What is a diopter?
A diopter (D) is the unit of optical power, defined as the reciprocal of focal length in meters. It indicates how strongly a lens or mirror converges or diverges light.
How do I use this calculator?
Enter the focal length in meters into the 'Focal length' field. The calculator will compute the optical power in diopters using the formula P = 1/f.
What does a positive or negative diopter mean?
A positive diopter indicates a converging lens (convex) or concave mirror, while a negative diopter indicates a diverging lens (concave) or convex mirror. The sign is determined by the sign of the focal length.

Tips & Common Mistakes

Tips

  • Ensure the focal length is in meters. If you have it in centimeters, divide by 100 before entering.
  • Use a negative value for diverging lenses or convex mirrors to get a negative diopter result.
  • Remember that optical power is the reciprocal of focal length, so shorter focal lengths yield higher diopters.
  • Double-check the sign of your focal length to get the correct sign for the optical power.

Common Mistakes to Avoid

  • Forgetting to convert focal length to meters, leading to incorrect diopter values.
  • Entering the focal length as a positive number when it should be negative for diverging elements.
  • Confusing the unit of focal length (meters) with the unit of optical power (diopters).

Last updated: August 13, 2026