Diopter: What is it? Uses, Connection in Series and Applications

It is a standard unit of measurement in eyeglass prescriptions. The best way to determine your necessary visual correction is to visit your optometrist.


The optometrist will be able to verify your vision and provide an accurate assessment of your needs.

A diopter can be a negative number (indicating myopia and a lens that minimizes things). Or it can be a positive number (indicating hyperopia and a magnifying lens).

If you already wear glasses to read, choose a monocle with the same diopter as your current reading glasses, rounding to the nearest 0.50 diopter. For example: if your reading glasses are +1.25, get a monocle that is +1.50. If your reading glasses are +2.75, get a monocle that is +3.00.

A diopter indicates how powerful a lens is to properly focus light on a person’s retina and is defined as “the inverse of a person’s focal length in meters.” The focal distance is the distance you need to see an object clearly for a detailed task, such as reading a book.

A diopter is a unit of measurement of the optical power of a curved lens or mirror, which is equal to the reciprocal of the focal length measured in meters.

Typically, the focal length of a lens uses millimeters as a unit, such as 50mm, 100mm, 8.9mm, and 71mm.


That is, dividing 1000 by the focal length (in mm) of a lens produces the diopter of that lens. For example, a 50 mm lens has a diopter of 1000/50 = 20, and a lens of 8.9 mm has a diopter of 1000 / 8.9 = 112.4.

Since the diopter of a lens is usually written as + d, the 50 mm and 8.9 mm lenses have diopters +20 and +112.4, respectively.

On the other hand, if we know the dioptric value of a lens, we can calculate its focal length. According to the previous formula, the focal length of a lens, given its diopter, is calculated as follows:

For example, if a lens has diopter +2, its focal length is 1000/2 = 500 mm, and a +4 diopter lens has a focal length of 1000/4 = 250 mm.

In this way, you can easily calculate the diopter of a lens from its focal length and the focal length of a foreground lens from its diopter value.

Therefore, it is a unit of reciprocal length. For example, a 3-diopter lens brings parallel light rays to focus at 1/3 of a meter. A flat window has an optical power of zero diopters and does not converge or diverge from light.

Diopters are also sometimes used for other reciprocals of distance, particularly radii of curvature and the vergence of optical beams. The use was proposed by the French ophthalmologist Ferdinand Monoyer in 1872, based on the previous use of the term dioptric by Johannes Kepler.

The main benefit of using optical power instead of focal distance is that the equation of the lens manufacturer has the distance of the object, the distance of the image, and the focal distance as reciprocals.

An additional benefit is that their powers add up approximately when relatively thin lenses are placed together. Therefore, a light two diopter lens near a thin 0.5 diopter lens produces almost the same focal length as a 2.5 diopter lens.

Although the diopter is based on the international metric system of units, it has not been included in the standard, so there is no international name or abbreviation for this unit of measure within the global system of units.

However, most languages ​​have borrowed the original name, and some national standards bodies such as DIN specify a unit name (diopter, diopter, etc.) and the derived unit symbol “DPT.”

Series connection of lenses

The use of lens power has an advantage if the total resistance of several lenses placed one behind the other must be calculated; the full force of the lens is equal to the sum of the resistances of the individual lenses (formula of the lens).

This is why ophthalmologists and opticians almost always work with the power of the lens.


The diopter (dot symbol) is a unit of force (“refractive power”) of a lens or mirror. A diopter is defined as 1 DPT = 1 m-1. The diopter can be considered a unit derived from the international system of units.

An application refers to glasses. Suppose someone is myopic and has a lens power of -2.5 pts for a long time. The focal point of your mirrors is then 40 cm in front of the lens.

If you also put presbyopia, you need a positive correction, for example, +2 DPT (focal point 50 cm behind the lens).

The total strength of your reading glasses (or the reading portion of a bifocal or multifocal glasses) becomes (-2.5 dpt) + (+2 dpt) = -0.5 dpt. If it ages, the reading correction required increases, for example: +3 dpt, so that reading lenses (-2.5 dpt) + (+3 dpt) = +0.5 dpt.

Another application is the conversion lens in photography. Here also, the effective focal length of a lens plus conversion lens is calculated by adding the intensities of the lens.

Therefore, a nearsighted person who needs a -1.00 diopter lens can see the objects a meter clearly, but anything farther away is blurred.

Someone with a measurement of -2.00 diopters requires a lens that is twice as powerful, so they can only see objects at a distance of up to 1/2 meter.

A lens of -3.00 would mean that the person can only clearly see a distance of up to 1/3 of a meter, and so on. Most myopic people are in the range of -1.5 to -7.00 diopters, which is considered mild to moderate.

On the other hand, a hyperopic person who needs a +1.00 diopter lens can see the objects in a meter, but anything closer is blurred. A +2.00 lens indicates that someone can see things 1/2 meter and beyond, but nothing more comparable.

Diopter is also a term used to identify the refractive capacity (bending of light) of a lens. A higher diopter indicates a greater refraction capacity and a greater magnification.