How to Measure Lenses Without a Phoropter

Many parts of the world lack access to adequate and affordable eye care. Yet measuring lens diopter (UK: dioptre) and cylinder is mathematically-precise. It is the reciprocal of the focal length in meters. Measurements are additive, so multiple lenses may be stacked together. With simple calculations and a measuring stick, anyone could look at a chart, fudge some numbers, and order cheap glasses online, check prescriptions, or notice if their eyesight is changing.*

Myopia and Astigmatism Calculator

SPH: CYL: Axis:

  1. Use a desktop computer or large tablet.
  2. Remove eyeware, or look through lens to be tested.
  3. Cover one eye while keeping head level.
  4. Move back from the above diagram until only some lines are clear.
  5. Click the line that is least blurry.
    • If two lines are clear, click between them.
  6. Use cloth tape or tape measure to get accurate measurements from eye to screen.
    • Measure within 1/2 cm. or 1/4 in. if possible for best results.
    • Farthest distance to screen when any line is clear:
    • Distance to screen when remaining lines are clear:
      (for miopia; look closer)

Reading glasses: For best results, consult a doctor, try on reading glasses at the store, or view the reading glasses tests at readers.com.

Disclaimer

*For educational purposes only. There is no substitute for visiting an eye professional every two years. They can spot eye diseases that if caught early could prevent serious problems later on. Lens calculations depend on the operator and the quality of measurements. As little as 1/4" makes a big difference.

This electronic document is provided with absolutely no warranty, neither expressed nor implied!