Can my eyes still be corrected?
What are diopters?
Diopter is a term used in ophthalmic optics and describes the unit of measurement of the refractive power for a spectacle lens or a contact lens. So it indicates how strongly a spectacle lens has to refract the light so that the person with ametropia can see clearly. The common abbreviation for diopter is “dpt”.
Negative diopter values represent nearsightedness, positive diopter values represent farsightedness. The higher the values, the worse the person can see without glasses and the more the correction must be made. The optician usually gives the diopter values in quarter diopter steps (0.25 D) and notes them on your glasses passport. For example, people with ametropia can hear: “I have +2 diopters in my right eye and +1.5 diopters in my left eye”.
Glasses pass with diopter values
From which diopter values do I need glasses?
From a diopter value of 0.5, it makes sense to compensate for your ametropia with glasses or contact lenses. Up to a strength of + -1 diopters one speaks of a slight ametropia that does not have to be continuously corrected with a visual aid. If the values are higher, it is recommended to always use the visual aid. From a value of + -5 D there is a strong ametropia.
For example, in order to read a book at a distance of 35 centimeters, a person with normal vision at the age of 45 needs reading glasses with approximately 0.75 diopters. Since the eye increasingly loses its elasticity with age (presbyopia), clear vision at close range becomes more and more difficult. Roughly, one can say that from this age onwards, a person's ametropia worsens by 0.75 diopters every 5 years. The following guidelines can be derived from this:
|AGE||DIOPTRIAL READING OF READING GLASSES|
Diopter values for nearsightedness and farsightedness
From -1 diopter it is a slight myopia and from -5 diopter it is a strong nearsightedness. Conversely, from a value of 1 dioptres onwards one speaks of a slight farsightedness and from 5 dioptres on of a strong farsightedness.
Diopter values for nearsightedness and farsightedness
Myopia persists in old age. With increasing presbyopia, however, you have the opportunity to read significantly better by removing the distance glasses. For a nearsighted person aged 55-60 years with distance -3 diopters, addition 2.0, reading glasses with -1.0 diopters would be required.
In the case of farsightedness, on the other hand, the presbyopia diopter values must be added. A farsighted person with 2 diopters would therefore need corrective glasses with 4.25 diopters (2 diopters + 2.25 diopters) at the age of 55-60 years.
How many diopters does a normal sighted eye have?
The human eye focuses the light with a value of about 60-65 diopters. The cornea accounts for about 40 diopters, the rest of the refractive power is exerted by the lens. Since the lens is flexible and controlled by the muscles, the human being is able to adapt the refractive power of the eye to the situation. When looking up close, the lens is compressed and thus thicker; when looking into the distance, the lens is elongated and thus thinner. This process is called accommodation.
Accommodation of the eye
With objects that are more than 6 meters away, the light rays hit the retina in parallel. The eye is relaxed - we see clearly (1). Without accommodation, the light rays would focus on closer objects behind the retina - we see blurred (2). Due to the accommodation, the eye lens bends and thereby increases the refractive power. The light rays are now bundled again in the retinal plane and we see sharply (3).
Can I calculate my diopter values myself?
With a little experience, myopic people can roughly estimate their ametropia themselves, but farsighted people cannot. A nearsighted person who can see up to a maximum of 1 meter without glasses needs a corrective lens of -1 diopters in order to be able to see clearly further than one meter. If sharp vision is only possible up to 0.5 meters, -2 diopters are necessary. The diopter value corresponds to the reciprocal value of the distance up to which one can see clearly.
If a nearsighted person wants to see sharply at a reading distance of 0.3 meters, this corresponds to a diopter value of 1 divided by 0.3 meters, i.e. approximately 3 diopters. However, if you have to hold the book 0.5 meters away from your eyes in order to read clearly, the actual visual acuity is 1 divided by 0.5 meters, i.e. 2 diopters. If you now subtract the actual value (2 D) from the desired value (3 D), you get the necessary refractive power with which the eye must be supported in order to be able to see clearly at the desired distance of 0.3 meters. Therefore, in our example, reading glasses with 1 diopter (3 D - 2 D) would be appropriate.
These own calculations are of course inaccurate. Badly fitted glasses can damage your eyes. You should therefore always have your glasses adjusted by your optician on site.
How does the optician determine my diopter values?
Special eye tests are offered by the optician, with which the required glasses values can be determined more precisely. In order to be able to carry out a reliable test on children and illiterate people, a test with the so-called Landolt rings is usually carried out. In this case, the test person is shown rings that are open on one side. If you can see the opening, you can see the ring clearly. If the view becomes blurred, the direction can no longer be given with certainty. As the rings get smaller from line to line, it becomes more and more difficult to see the ring clearly. The line in which the test subject has to stop or make their first mistakes indicates the eyesight. The initial assessment can be checked with the aid of apparatus in which different lens thicknesses can be easily set. The lenses are adjusted until a visual acuity of approximately 1.0 can be achieved in the test.
Landolt eye test
The usual equipment of an optician allows him to reliably determine dioptre values between -19 and +16 diopters. With appropriate adjustments, a value of up to + -20 diopters can theoretically be achieved. Thanks to modern ophthalmology, however, values in the double-digit diopter range are rarely found. Eyes that have poor eyesight are changed with the help of laser surgery or other treatments so that low-strength visual aids are sufficient for a clear field of vision.
Are the diopter values the same for glasses and contact lenses?
Up to a value of 4 diopters, the strength of soft contact lenses and glasses does not differ. From a value of 4 diopters, however, the contact lens values are lower for minus and higher for plus than for the glasses values. In the case of hard lenses, there is also the fact that the cylinders usually also change. Therefore, if the eyesight changes, the optician should determine both the glasses and the lens values.
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