Table of Contents
What is a Diopter?
A diopter (D) is the unit of measurement for the optical power of a lens or curved mirror. It is defined as the reciprocal of the focal length in meters. 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. Converging (convex) lenses have positive diopter values, while diverging (concave) lenses have negative values.
The diopter system is particularly useful in optometry and ophthalmology because the powers of thin lenses in contact simply add together. An eye doctor prescribing corrective lenses can combine the powers of different corrections (sphere, cylinder, prism) by addition, making lens design straightforward. This additive property is why the diopter is the standard unit for optical prescriptions worldwide.
Formula
Vision Correction
- Myopia (nearsighted): Negative diopters, diverging lenses (-1 to -10 D typical)
- Hyperopia (farsighted): Positive diopters, converging lenses (+1 to +6 D typical)
- Presbyopia: Reading glasses, typically +1 to +3 D added power
- Astigmatism: Cylindrical correction with axis angle
Common Lens Powers
| Focal Length | Power (D) | Application |
|---|---|---|
| 1.0 m | +1.0 | Weak reading glasses |
| 0.5 m | +2.0 | Standard reading glasses |
| 0.1 m | +10.0 | Magnifying lens |
| 0.017 m | +59 | Human eye (total) |
FAQ
What does my eyeglass prescription mean?
The sphere value (SPH) is the main correction in diopters. Negative values correct myopia, positive values correct hyperopia. The cylinder (CYL) corrects astigmatism. The ADD value is extra positive power for reading in progressive or bifocal lenses. Higher absolute values mean stronger correction is needed.
Can I add diopters of lenses not in contact?
For lenses separated by distance d, the combined power is P1 + P2 - d*P1*P2. The simple addition rule only applies when lenses are in contact (d=0). Separation significantly affects the combined focal properties of the lens system.
What is the diopter power of the human eye?
The human eye has a total optical power of about 59 diopters, with the cornea contributing about 43 D and the crystalline lens about 16 D. The lens can change its power through accommodation (changing shape), adding up to about 12 D in young people, decreasing with age to near zero by age 60.