Diopters: The unit of measurement for refractive power & what the values mean (diopter table)
Have you ever thought about what the numbers on your spectacle pass mean? Or why the diopter values for contact lenses and glasses are different? Diopters are more than just a number – they describe how severe your defective vision is and how it can be corrected. In this blog, we take a deep dive into the world of diopters:
What exactly does the diopter value say about your vision? How do short-sightedness, long-sightedness and astigmatism differ in their values? And how is it actually measured whether you need glasses and contact lenses or perhaps even laser eye treatment? What are the diopter limits for laser eye surgery / lens surgery?
Here you will find the answers – diopters explained simply!
What are diopters? – The unit of measurement for defective vision
Diopters (abbreviated as dpt) are the unit of measurement used to describe the refractive power of optical lenses. In the context of the eye and defective vision, diopters indicate how strongly a spectacle lens or contact lens must refract light in order to correct vision. The term diopter therefore describes the ability of a lens to focus light rays in such a way that they produce a sharp image on the retina of the eye.
An eye with normal vision has a refractive power of around 60-65 diopters, as the lens of the eye and the cornea refract the light so that it is focused on the retina. However, if the refractive power of the eye is too weak or too strong, this results in short-sightedness(myopia) or long-sightedness(hyperopia). This is where negative values for myopia and positive values for hyperopia come into play.
A lens with a power of 1 diopter changes the focal point of light by exactly one meter. Higher diopter values, such as 13 diopters, indicate a more severe visual defect, which often needs to be corrected with thicker glasses or specially adapted contact lenses.
Diopter table: An overview of the diopter values
Here is a diopter table that provides an overview of the typical diopter values for the correction of myopia, hyperopia and other visual defects:
| Diopter value (dpt) | Poor eyesight | Description |
|---|---|---|
| 0 dpt | Normal vision (emmetropia) | The eye has normal vision and does not require correction. Sharp vision is possible. |
| +0.25 to +2 dpt | Slight farsightedness(hyperopia) | Slight difficulty focusing on nearby objects. Can be corrected with reading glasses. |
| +2 to +5 dpt | Medium farsightedness | Significant loss of near vision, often also difficulties in reading without a visual aid. |
| +5 dpt and more | Severe farsightedness | Severe near vision problems that need to be corrected with strong spectacle lenses or contact lenses. |
| -0.25 to -3 dpt | Slight short-sightedness (myopia) | Slight problems seeing distant objects clearly. Glasses or contact lenses with negative values necessary. |
| -3 to -6 dpt | Medium myopia | Vision at medium and long distances is blurred. Stronger correction is required. |
| -6 dpt and more | Severe myopia | Severe short-sightedness, objects are very blurred in the distance. High diopter values often require special lenses. |
| ±0.25 to ±2 dpt | Astigmatism(curvature of the cornea) | Distorted vision due to an uneven cornea. Can occur with both short-sightedness and long-sightedness. |
| +2 to +3 dpt | Presbyopia (presbyopia) | Difficulty with near vision due to the natural ageing process. Often needs reading glasses. |
This table provides a general overview of how diopter values affect vision. Values from +20 diopters or -20 diopters are very rare and indicate extreme visual defects, which are often corrected by special visual aids or even surgical procedures such as laser eye surgery.
How does defective vision develop? Relationship between diopters and visual acuity
Visual defects occur when the eye is unable to focus the incoming light correctly on the retina. This is usually due to a malformation of the eyeball, an uneven cornea or an altered refractive power of the lens.
Short-sightedness and long-sightedness: myopia and hyperopia explained
With short-sightedness, the eyeball is too long or the refractive power of the eye is too strong, so that light is focused in front of the retina. As a result, distant objects appear blurred, while near objects are seen clearly. The correction is made using negative diopters, which redirect the light so that it hits the retina correctly again.
With long-sightedness, the eyeball is too short or the refractive power is too weak. This means that the light is focused behind the retina, making it difficult to see close up. Positive diopter values help to focus the light more strongly and thus improve vision.
Astigmatism and the diopter value
Astigmatism is caused by an unevenly curved cornea or lens, which causes light to be refracted in different directions. This leads to distorted or blurred vision, both near and far. Astigmatism and short- or long-sightedness often occur together and require special visual aids.
Presbyopia: when the lens becomes inflexible
With increasing age, the lens loses flexibility, which means that the eye has problems focusing on objects close up. This so-called presbyopia usually occurs from the age of 40 and is corrected with reading glasses or special progressive lenses.
Measurement of diopters: What the optician and ophthalmologist do
Measuring diopters is an important step in determining defective vision and prescribing the right visual aid. Both opticians and ophthalmologists use special procedures and devices to determine visual acuity. Here are the most important steps:
- Medical history and preliminary examination
The optician or ophthalmologist will ask about visual problems and the use of visual aids. Possible complaints such as headaches or blurred vision are also asked about. - Autorefractometer
A computerized device automatically measures how strongly light is refracted in the eye. This first measurement gives a rough estimate of the diopter values. - Eye test
An eye chart is used to display letters or symbols in different sizes. The patient reads these at different distances to test their visual acuity. - Subjective refraction
In a further step, the exact dioptric correction is determined by changing lenses. The patient gives feedback on which lenses provide the sharpest vision. - Measurement of astigmatism
A special cylindrical lens is used to detect possibleastigmatism. This checks whether the image is distorted and how much correction is required. - Determination of near and distance vision
Both short-sightedness and long-sightedness are tested. The strength of the visual aid for near vision (reading) and distance vision (e.g. driving) is determined. - Glasses pass or contact lens prescription
After the measurement, the patient receives either a glasses pass or a prescription for contact lenses, on which the dioptric values determined and, if applicable, further information on visual acuity are noted.
Correcting defective vision: Glasses, contact lenses & laser eye surgery / lens surgery
Diopter value for laser eye surgery
In laser eye surgery, the diopter value plays an important role in determining whether someone is a suitable candidate for the procedure. Procedures such as Femto-LASIK, Trans-PRK or ReLEx SMILE / SmartSight can permanently change the refractive power of the eye by reshaping the cornea so that the light falls correctly on the retina again. The laser corrects both short-sightedness and long-sightedness and can even eliminate astigmatism (curvature of the cornea).
- Short-sightedness: As a rule, up to -10 diopters can be corrected.
- Farsightedness: Depending on the procedure, up to +6 diopters can be treated for farsightedness.
- Astigmatism: Corrections of up to around 5 diopters are also possible for astigmatism.
However, depending on the level of diopters, the decision as to whether laser treatment is possible depends on the condition of the cornea and other factors.
What to do if your eyesight is too poor: lens surgery as an alternative
Traditional laser eye surgery is often no longer sufficient for very severe defective vision, as there are limits to the treatment. If the diopter value is too high or the cornea is too thin, lens surgery is an excellent alternative. Procedures such as ICL lens implantation or lens exchange (also known as refractive lens exchange) are used here. These options are particularly suitable if short-sightedness or long-sightedness exceeds the usual limits for laser treatment.
ICL lens implantation (implantable contact lenses)
ICL lens implantation is a minimally invasive method in which an additional, specially made lens is inserted into the eye to correct vision. This lens is implanted in front of the patient’s own lens and behind the iris so that it is not visible from the outside. The great advantage of this method is that it can also be used for very high diopter values:
- Myopia: Up to -20 diopters.
- Farsightedness: Up to +10 diopters.
ICL lenses are particularly suitable for people for whom laser eye surgery is not possible, as no cornea is removed and the natural structure of the eye is therefore preserved.
Lens exchange (refractive lens exchange)
Lens replacement involves replacing the natural lens with an artificial one, similar to cataract surgery . This method is particularly suitable for people with very high diopter values that cannot be successfully treated with either a laser or an ICL lens. Lens exchange offers the possibility of permanently correcting extremely severe visual defects:
- Short-sightedness and long-sightedness: Diopter values of +20 diopters or -20 diopters and higher can also be compensated for by lens replacement.
- This method is also particularly suitable for presbyopia, as the inserted artificial lens also improves near vision.
Advantages of lens surgery
- High diopter values that are too extreme for laser eye surgery can be corrected.
- The interventions are permanent and can last a lifetime.
- The natural visual process is supported by the new lenses without having to change the cornea.
- A safe and proven method for short-sightedness, long-sightedness and astigmatism.
Diopter value for glasses
With glasses, the diopter value is determined by the strength of the lenses. The lenses correct the refractive error of the eye by redirecting the light so that it is correctly focused on the retina. The advantage of glasses is that they are easy to adjust if your eyesight changes over time.
- Negative diopters (e.g. -2, -5 diopters) correct short-sightedness by scattering the light and shifting the focal point backwards.
- Positive diopters (e.g. +3, +6 diopters) focus the light and help to shift the focal point forwards in the case of long-sightedness.
Diopter value for contact lenses
Contact lenses work on the same principle as spectacle lenses, as they correct the refractive error of the eye. However, as the lenses sit directly on the cornea, there is a difference in the diopter values required.
Contact lenses often have a slightly lower diopter value than spectacles. This is because the spectacle lenses sit at a certain distance from the eye (approx. 12 mm), while the contact lenses rest directly on the cornea . This distance influences the optical effect of the lens, so that the diopter value of the glasses must be slightly higher to achieve the same correction.
Example:
- If you need a value of -3 diopters for glasses, the value for contact lenses could be slightly lower, for example -2.75 diopters.
