What does diopter mean?
A diopter is the standard unit for measuring the refractive power of a lens or curved mirror. Defined as the reciprocal of the focal length in metres, it allows optometrists and optical scientists to describe how strongly a lens bends light: a lens that focuses parallel light at one metre has a power of one diopter, while a stronger lens with a shorter focal length has a correspondingly higher value. The term is central to everyday eye care, appearing on spectacle and contact lens prescriptions, where positive values correct farsightedness and negative values correct nearsightedness. It also surfaces in photography, both as a rating for close-up attachment lenses and in the diopter adjustment dial that lets users fine-tune their viewfinder focus. Though technical in register, the word is familiar to anyone who wears glasses. British English usually spells it 'dioptre'; American English prefers 'diopter'.
A unit of refractive power of a lens or prism, equal to the reciprocal of its focal length in metres.
"Reading glasses of two diopters magnify objects held at about fifty centimetres."
Usually plural when discussing prescriptions ('3.5 diopters'); singular forms appear in phrases like 'half a diopter stronger'.
In photography, a supplementary lens or viewfinder focusing adjustment whose strength is rated in diopters.
"He attached a two-diopter close-up lens to capture detail shots of the insects."
This sense always refers back to the optical unit; it does not introduce a separate physical object distinct from lens power ratings.
Regular plural. Prescriptions commonly use decimal values such as '-1.25 diopters'; the singular appears mainly after fractions, e.g. 'half a diopter'.
"His prescription changed by only a quarter of a diopter, yet he noticed the difference immediately."
A diopter is just the flip side of focal length: a lens with a one-metre focal length is 1 diopter, half a metre is 2, and your optometrist has been doing that division every time they write your prescription.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of diopter
The word 'diopter' derives from the Greek 'dioptra', the name of an ancient sighting and surveying instrument, formed from 'dia-' meaning 'through' and a root related to 'ops' or 'optos', meaning 'eye' or 'sight'. The French astronomer and mathematician Joseph de Mendoza y Ríos proposed the term in the late eighteenth century as a unit of lens power, and it entered scientific English around the early nineteenth century. It shares its Greek root with words such as 'optic', 'autopsy', and 'synopsis', all of which involve seeing.
Related word forms
How diopter is actually used
Primarily a technical term used in optometry, ophthalmology, photography, and optics; it is uncommon in casual conversation outside these fields. British English typically spells it 'dioptre', while American English uses 'diopter'. Pluralization is regular ('diopters'/'dioptres'), though prescriptions often appear as decimals such as '-2.50'.
Easily confused with diopter
A diopter is the modern unit of lens power, while the dioptra was an ancient Greek surveying and astronomical sighting instrument.