Optical Calculators

Equivalent Power Calculator

Calculate the true combined power of two separated lenses, or of one thick lens. Choose Two Lens System or Thick Lens, enter the powers and spacing, and you'll get the net equivalent power.

Two lenses held apart, or one thick lens, deliver less power than their powers simply added together, because light bends between the surfaces. This calculator uses Gullstrand's equation to account for that gap.

It has two modes:

  • Two Lens System: for two separate lenses, such as a magnifier held in front of glasses, a telescope, or a trial lens stack. Enter both powers, the distance between them, and the index of the medium between them (1.000 for air).
  • Thick Lens: for a single lens. Enter the front and back surface powers, the center thickness, and the material's index (1.498 for CR-39, 1.586 for polycarbonate).

Equivalent power formula: Equivalent Power = P1 + P2 − (d / n) × P1 × P2, where d is the distance between the lenses and n is the index of refraction

Use it to predict the real power of multi-lens low vision devices, high-power microscopes and thick spectacle lenses.

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