What Is a Diopter?

A diopter measures the optical power of a lens, expressed in the form of a reciprocal of the focal length. This is measured in meters, even in nations that do not use metric measurement, and is widely used in optics. For example, a five diopter lens brings light to focus at a distance of 1/5 meter. The higher the measurement, the closer the focal length. This can also be expressed as a negative, like -2 diopters, indicating a greater distance.

In the development of curved lenses and mirrors, the shape determines where the focus lies. For corrective lenses like glasses used to compensate for vision problems, a care provider can determine the appropriate number of diopters needed to correct someone’s vision. The lenses can increase or decrease the focal length, depending on the nature of the vision impairment, to allow the patient to see more clearly. Someone who needs a 2 diopter lens, for example, is slightly farsighted, and needs the lens to shorten focal length so light will hit the right part of the eye.

Diopters are also important for magnification in equipment like microscopes, and focus in optical devices. Telescopes, binoculars, and other equipment people look through in order to view phenomena need to be focused appropriately for the user. Many have a diopter adjustment to allow people to do so. In other cases, the diopters can be important for machine-made measurements and activities like focusing lasers used in research.

Although this unit of measurement has not been adapted as part of the standard system, it is very widely used around the world. Optics researchers, technicians, and developers commonly use the diopter to express optical power because it is familiar and well understood. Laypeople may also be familiar with the concept, either because they were corrective lenses or use devices like cameras that may require diopter adjustment.

This term can be abbreviated d or dpt, depending on preference. In the case of lenses for vision, prescriptions are commonly expressed in quarters, like +1.25 or -3.75. These can allow for increased precision, ensuring the greatest possible degree of correction for the patient. The level a patient needs can vary between eyes, an important consideration with high powered prescriptions in particular. Generic reading glasses usually offer the same level of correction through both lenses, and patients may want to try several powers before purchasing to make sure they have the best available option.