Free · Optics and light calculators 🔍

Positive Lens Power & Focal Length

Convert positive focal length to positive optical power in diopters and solve the inverse.

  • Formula & worked example
  • Private in your browser
  • No signup
Go to tool

Calculator inputs

Loading calculator…

How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

A positive focal length of 0.5 m corresponds to +2 diopters. A 250 mm focal length corresponds to +4 diopters.

Optical power in diopters = 1 / focal length in meters

Worked example

Enter these known values and leave the other values blank.

Positive focal length
0.5 m
Positive optical power in diopters
2 D

Assumptions and limitations

  • This retained model accepts positive focal length and positive optical power only. Diverging lenses with negative focal length are outside its supported input domain.
  • One diopter is one inverse meter. Selecting millimeters or centimeters changes the input scale without changing the physical focal length.
  • This is an optical unit relationship, not an eyeglass prescription or a complete multi-element lens model.

Common questions

Does larger positive power mean a shorter focal length?

Yes. For example, doubling from +2 to +4 diopters halves positive focal length from 0.5 m to 0.25 m.

Can I use a negative diopter value here?

This calculator preserves the positive-only source domain. It does not cover diverging lenses.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

Explore optics and light calculators 🔍 or browse all physics calculators.