Diopter

A diopter is a unit of optical power equal to the reciprocal of focal length in meters. It quantifies how strongly a lens or curved mirror converges or diverges light.

Optical power: The reciprocal of a lens's focal length, describing its ability to converge or diverge light. A diopter is the standard unit used to express optical power.
Eyeglass prescription: A specification of corrective lens powers and other parameters used to make eyeglasses. Its sphere and cylinder values are commonly stated in diopters.
Refraction: The change in direction of a wave as it passes between materials with different refractive indices. Refraction at lens surfaces produces the focusing measured by optical power.
Prismatic power: The ability of a prism to deviate light, commonly measured in prism diopters. It is a different optical quantity from lens power measured in diopters.
Focal length: The distance from an optical system's principal plane to the point where parallel rays converge or appear to diverge. Taking its reciprocal in meters gives a lens's power in diopters.
Refractive error: An optical focusing condition in which the eye does not form a sharp image on the retina. Diopter values describe the correction needed for many refractive errors.
Refractive index: A dimensionless measure of how much light slows in a material relative to vacuum. It helps determine how strongly a lens bends light and thus its diopter power.
Magnification: The ratio of an image's size to an object's size. Diopters describe focusing power, not the size change produced by an optical system.
Converging lens: A lens that brings parallel incident rays toward a real focus. Its positive focal length corresponds to positive diopter power.
Myopia: A refractive error in which distant objects focus in front of the retina. Minus-diopter lenses shift the focus farther back onto the retina.

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