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Lens Power (Dioptre) Calculator

Work out lens power (dioptre) instantly with clear inputs, formula shown and shareable results.

About Lens Power (Dioptre) Calculator

Lens power in dioptres is the reciprocal of the focal length in metres, so a 0.25 m converging lens is +4 D and a diverging lens of the same strength is -4 D. Optometrists use dioptres because powers of lenses in contact simply add, which makes prescriptions and trial-lens stacking straightforward.

Formulas

Lens power

P = 1 / f, with f in metres

Sign convention

converging lenses positive, diverging negative

Worked example

With the default values below, Lens Power (Dioptre) Calculator returns Lens Power of 4.000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Focal length
0.25 m
Lens typeone of Converging (positive), Diverging (negative)
Converging (positive)

Results

Lens Power
4.000 D
Signed Focal Length
25.00 cm
Magnitude Of Power
4.000 D
Optical Role
Converging - corrects hyperopia (long sight)

Step by step

  1. 1Values used
  2. 2Lens power
  3. 3Sign convention
  4. 4Lens Power= 4.000 D
  5. 5Signed Focal Length= 25.00 cm
  6. 6Magnitude Of Power= 4.000 D
  7. 7Optical Role= Converging - corrects hyperopia (long sight)

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why do opticians use dioptres?

Because powers add. Stacking a +2 D and a +1 D lens gives +3 D, whereas focal lengths would need reciprocals combined.

What does a -3 D prescription mean?

A diverging lens with a 33 cm focal length, correcting short sight so distant objects focus on the retina.

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