Dioptria

Dioptria, or diopter, is the unit of measurement for the optical power of a lens or optical system. It is defined as the reciprocal of the focal length in meters, so a lens with a focal length of 1 meter has a power of 1 diopter (D). The diopter is a non-SI unit widely used in optics and ophthalmology, abbreviated as D. The term derives from the Greek dioptra and was popularized in ophthalmology in the 19th century, with Ferdinand Monoyer commonly associated with its introduction.

In eye care, diopters quantify the refractive error of the eye and the corrective strength of eyeglasses or contact lenses. The total refractive power of a normal, young eye is typically around 60 D, with the cornea contributing about 40 D and the crystalline lens around 20 D. Refractive errors are expressed in diopters: myopia (nearsighted) uses negative diopters, hyperopia (farsighted) uses positive diopters, and astigmatism involves cylindrical diopters along specific meridians. Prescriptions may also specify axis and cylinder power to correct uneven curvature.

Measurement of refractive power employs various devices and methods, including phoropters, retinoscopy, and autorefractors, often during a refraction test to determine the best corrective lens strength. The resulting prescription indicates the spherical component (SPH), cylindrical component (CYL), and axis for astigmatism, all expressed in diopters.

Diopters are also used outside clinical optics, such as in close-up lenses for photography, where higher diopter values indicate stronger magnification for near focusing. In all contexts, the diopter provides a convenient, inverse relationship between focal length and lens power.