Diopter refers to the measure of the optical power of a lens and is determined by the focal length. The diopter is always measured in meters and is expressed as the inverse of the focal length. For instance, a lens that is four diopter makes light focus at a 1/4 meter distance. A higher measurement of diopter means a closer focal length. Alternatively, the diopter can also be a negative number such as -2, to indicate a larger distance. This unit of measurement can be abbreviated as either dpt or d.

One of the most common application of diopters is in the field of optics. For instance, in the making of corrective lenses that are used by people with vision problems, the number of diopters that is required to correct the vision is determined. The higher the value of diopter, the stronger the correction and lens. For prescription glasses, diopters are normally expressed in quarters such as +2.25 or -1.75, for a higher level of precision. Diopters are also measured carefully for each eye because the needs of the patient may be different for each eye.

Aside from its application in corrective glasses, this is also important for pieces of equipment such as microscopes, binoculars, telescopes, as well as other related things that people use to witness different types of phenomena. These pieces of equipment have a diopter adjustment that will enable people to focus and magnify appropriately. Another important application of diopters is in the focusing of lasers that are used in research.

Although this is not yet officially a part of the standard system, this unit of measurement is used all over the world by technicians and researchers in the field of optics. Common people may also be familiar with the term because they use corrective glasses or gadgets that have diopter adjustments.