Diopter Calculator
Convert between focal length and optical power in diopters.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Enter the focal length and select the unit (meters or centimeters). The calculator instantly shows the optical power in diopters. Negative focal lengths indicate diverging lenses.
About Diopter
The diopter (D) is the standard unit for optical power, defined as the reciprocal of focal length in meters. It is the primary unit used in optometry and ophthalmology to specify lens prescriptions. Positive diopters indicate converging lenses (used for farsightedness), and negative diopters indicate diverging lenses (used for nearsightedness). Multiple lenses in contact have a combined power equal to the sum of their individual diopter values.
Frequently Asked Questions
What is a diopter?
A diopter is the unit of measurement for the optical power of a lens. It equals the reciprocal of the focal length in meters. A lens with a focal length of 0.5 m has a power of 2 diopters.
Can diopters be negative?
Yes. Diverging (concave) lenses have negative focal lengths and negative diopter values. Converging (convex) lenses have positive values.
How are diopters used in eyeglasses?
Eye prescriptions are written in diopters. Nearsighted people need negative diopters (diverging lenses) while farsighted people need positive diopters (converging lenses).