Diopters: The Unit of Vision

In cases of visual impairments, the refractive power of the eye is disrupted, causing objects to appear blurry. There are various types and degrees of vision defects, all measured in diopters. A diopter (abbreviated as dpt) is the unit of measurement for the optical system.

What is a Diopter?

A diopter is a unit of measurement for the refractive power (also referred to as the "refractive value") of the human eye. It indicates how strongly the eye bends light as it passes through the lens and onto the retina. The greater the deviation of the light ray from the standard value of 0, the higher the diopter value.

Normal-sighted eyes have 0 diopters. In cases of farsightedness, nearsightedness, or vision impairments caused by astigmatism, this measurement deviates above or below zero. Diopters are recorded in prescriptions from eye doctors or opticians in increments of 0.25.

• Nearsightedness is measured in negative diopters, indicated by a minus symbol. For example, a glasses prescription might read sph -5.25 dpt (or spelled out as diopters).

• Farsightedness is measured in positive diopters, shown with a plus symbol. For instance, an eye doctor might write sph +2.57 dpt on a prescription.

• Astigmatism is also measured in diopters. Depending on the notation, the value might have either a plus or minus symbol, such as cyl (or zyl) -3.50 dpt.

Diopters follow their own scale, starting from 0 and progressing in increments of 0.25 dpt, either into the positive or negative range. Opticians document the respective diopter values for the left and right eye in quarter-diopter steps on a glasses prescription.

Diopter Values on the Glasses Prescription

The diopter values noted on the glasses prescription allow the optician to fit glasses or contact lenses. The prescription includes a series of numbers:

• Spherical Value: The first number is called the spherical value or SPH. It represents the number of diopters needed to correct distance vision. A negative value indicates nearsightedness, while a positive value indicates farsightedness.

• Cylinder: The second number is the cylinder value. It indicates the correction for astigmatism, in case there is a corneal irregularity.

• Axis: The third number, given in degrees, is the axis. It shows the orientation of the astigmatism and specifies the direction in which the lens or laser should correct it.

If only the spherical value is listed on the prescription, with no cylinder or axis values, it means that there is no astigmatism present.

When is a Glasses Prescription Needed?

Not every refractive error requires corrective eyewear. In general, a refractive error of up to +/- 1 diopter is considered mild. If the refractive error is below this threshold, corrective eyewear is not necessarily needed on a permanent basis. Often, glasses or contact lenses may only be worn for activities like driving or watching television. Whether you need eyewear for road traffic is determined during your driver’s license vision test.

In general, the more severe the refractive error, the more likely it is that corrective eyewear such as glasses or contact lenses will be required to prevent everyday disturbances.

Traditional aids for correcting refractive errors include glasses or contact lenses. Laser eye surgery can provide a permanent solution, provided you meet the necessary criteria for the procedure. If the surgery is successful, corrective eyewear is typically no longer required.

Can Diopter Values Change?

A refractive error can progress and change over time. Currently, this progression is difficult to stop or slow down. However, certain habits seem to promote an increase in diopter values. These include factors such as work environment, growth, hormones, medications, or ageing.

People whose jobs involve spending significant time in front of screens and in enclosed spaces may experience worsening vision. On the other hand, spending time outdoors and limiting screen time at close distances can promote eye health and preserve vision.

Diopters in Myopia (Nearsightedness)

In a nearsighted eye, incoming light forms a focal point before reaching the retina. The larger the distance between the focal point and the retina, the more severe the nearsightedness, and the more diopters are needed to correct the image.

To correct myopia, a diverging lens is required. This type of lens has a concave shape and scatters light rays in front of the eye’s lens, shifting the focal point backward until it reaches the retina. Severe myopia, often referred to as myopia magna, starts at -6 diopters and can, in rare cases, reach as low as -25 diopters.

Diopters in Hyperopia (Farsightedness)

In farsightedness, the focal point is formed behind the retina. To bring objects into sharp focus on the retina, a converging (convex) lens is required to shift the focal point forward. The larger the distance between the focal point and the retina, the higher the correction needed, and the stronger the farsightedness. Glasses or contact lenses with higher diopters are needed for correction.

For mild farsightedness, reading glasses with lower diopter values may be necessary for reading small text, while normal-sized letters in a book or newspaper can still be easily read. Unlike nearsighted individuals, those with farsightedness often notice presbyopia (age-related farsightedness) earlier, as they struggle to focus on near objects. Hyperopia can, in rare cases, reach +13 diopters.

Diopters in Both Near- and Farsightedness

Myopia and hyperopia can sometimes occur simultaneously, where one eye is nearsighted and the other is farsighted. If the difference in diopters exceeds two diopters, this is known as anisometropia.

This differs from presbyopia, which is not a refractive error but an age-related condition affecting the eye's lens.

Diopters in Astigmatism (Corneal Astigmatism)

Both nearsightedness and farsightedness can occur alongside astigmatism, a condition where the cornea (the front layer of the eye) is slightly misshapen, causing light to be refracted unevenly. This leads to blurry vision, as the light does not focus properly on the retina.

Astigmatism is corrected using toric lenses, and depending on the notation, it can be indicated with either a positive or negative symbol.

Diopters in Presbyopia (Age-Related Farsightedness)

As mentioned earlier, presbyopia is an age-related condition affecting the lens of the eye. As we age, the lens loses its elasticity, making it harder to focus on near objects. Symptoms usually begin to appear around the age of 45.

Presbyopia is corrected with an additional lens power for near vision, known as an “addition”. This is usually marked with a plus symbol on the prescription, typically ranging between +1.00 and +2.50 diopters (in rare cases, it can go up to +2.75 or +3.00 diopters). People with presbyopia often use reading glasses, bifocal, or progressive lenses, or contact lenses.

Determining Dioptrien Values

Dioptrien values are measured during an eye examination. A series of tests are performed to determine if there is a refractive error and to what extent it affects vision. The tests involve various devices to assess the ability to recognise letters, images, or symbols on a chart placed at a specific distance. These tests help determine the visual acuity (visus) and the diopter level.

Snellen Visual Test

The Snellen visual test, developed by the ophthalmologist Herman Snellen, is a subjective test in which the patient reads letters from a Snellen chart. The test checks how well the person can recognise letters placed at a specific distance.

One eye is covered to measure the vision in each eye separately. For example, to measure the left eye, the right eye is covered. The person is then given different lenses to test which one provides the clearest vision.

Landolt Visual Test

Another method is the Landolt visual test, created by Swedish ophthalmologist Edmund Landolt. Instead of letters, vision is tested with Landolt rings, which have openings at different points. The rings get smaller as they go down the chart. The person must determine which direction the opening of the ring is facing. Depending on the smallest size the person can clearly identify, myopia (nearsightedness) can be determined.

Autorefractor

In addition to a standard eye test, an autorefractor can be used. During this test, the person rests their chin on a special device and simply looks at an image inside the machine. The image is automatically adjusted to be sharp or blurry. The device then automatically calculates the refractive value by determining when the image is focused on the retina, providing a precise diopter measurement for each eye.

Origin of the Diopter

The term "diopter" originates from the French ophthalmologist Ferdinand Monoyer and was quickly adopted by the international community as the unit of measurement for refractive errors. The term was first used officially in relation to the refractive power of the eye at an ophthalmologist conference in Brussels in 1875. As a result, the French word dioptrie was incorporated into most languages.

Monoyer also developed a visual acuity test, known as the Monoyer scale. This scale is still used worldwide today.