What is a diopter?

A diopter (D) is the unit of measurement for the optical power of a lens. It represents how strongly a lens converges or diverges light. The diopter value is calculated as the reciprocal of the focal length measured in meters: D = 1/f. A lens with a focal length of 1 meter has a power of 1 diopter, while a lens with a focal length of 0.5 meters has a power of 2 diopters.

Positive diopter values indicate converging (convex) lenses that bring light rays together to a focal point. These lenses are thicker in the middle and are used to correct farsightedness (hyperopia). Negative diopter values indicate diverging (concave) lenses that spread light rays apart. These lenses are thinner in the middle and correct nearsightedness (myopia).

How do diopters relate to vision correction?

When you visit an optometrist, your prescription is written in diopters. A prescription of -2.00 D means you need diverging lenses to correct myopia, while +1.50 D indicates converging lenses for hyperopia correction. The higher the absolute diopter value, the stronger the lens power needed. Understanding this relationship helps explain why stronger prescriptions result in thicker lenses and why lens material choice matters for higher corrections.

Tool description

This diopter calculator provides bidirectional conversion between focal length and diopter values. Enter a focal length in millimeters, centimeters, or meters to calculate the corresponding optical power in diopters, or input a diopter value to find the equivalent focal length. The tool automatically identifies the lens type (converging or diverging) and indicates which vision condition it would correct.

Examples

Focal length to diopter:

Focal Length Diopter Lens Type
500 mm +2.00 D Converging (convex)
250 mm +4.00 D Converging (convex)
100 mm +10.00 D Converging (convex)
1 m +1.00 D Converging (convex)

Diopter to focal length:

Diopter Focal Length Vision Correction
+1.00 D 1000 mm (1 m) Farsightedness
+2.50 D 400 mm (40 cm) Farsightedness
-3.00 D 333.33 mm Nearsightedness
-5.00 D 200 mm (20 cm) Nearsightedness

Features

  • Bidirectional conversion: Switch between focal length to diopter and diopter to focal length calculations
  • Multiple unit support: Input focal length in millimeters, centimeters, or meters with automatic conversion
  • Lens type identification: Automatically determines if the lens is converging (convex) or diverging (concave)
  • Vision correction indicator: Shows whether the lens corrects for nearsightedness or farsightedness
  • Multi-unit output: Displays focal length results simultaneously in mm, cm, and m for convenience

Use cases

  • Optometry students learning the relationship between focal length and lens power for exam preparation
  • Photographers calculating the optical power of camera lenses or close-up diopter filters for macro photography
  • Eyeglass wearers understanding what their prescription diopter values mean in practical focal length terms
  • Physics teachers demonstrating optics concepts with real-world calculations for classroom instruction
  • Optical engineers quickly converting between focal length specifications and diopter measurements during lens design