Meaning
Hardly any other word is used so often by opticians, but hardly anyone knows exactly what the term means. Dioptre is a unit of measurement used to indicate the strength with which a lens refracts light. Dioptre is therefore also an indicator of the degree of ametropia, since the power of the glasses takes over the refraction that the ametropic eye does not perform correctly itself.
Plus values in the dioptres correspond to long-sightedness, minus values to short-sightedness. Whether the dioptre value is positive or negative, the higher the dioptre value, the higher the ametropia. This is because the refractive power of the lens is then stronger. At the optician’s, the refractive power of the lenses for the spectacles is usually given in quarter dioptre steps (0.25 dioptre steps) and only very rarely are there smaller gradations.
Adjusting the diopters
For a short-sighted person, everything becomes blurred from a certain maximum distance and only in the vicinity is he able to see clearly without glasses. With the maximum distance, which can be determined by the patient himself, a nearsighted person can determine the dioptre number of the corrective lens quite precisely. Example 1: A nearsighted person can see clearly without glasses up to a maximum of one meter.
Therefore, he needs a lens with the value “- 1 diopter” to see at a distance. Example 2: If a near-sighted person can see clearly up to a maximum of 50 cm, he or she will need minus two diopters. Example 3: A near-sighted person with a maximum visual acuity of 33 cm will need a lens with minus three diopters.
These self-experiments are of course inaccurate and should be carried out precisely and professionally by an optician when fitting lenses. Opticians have precise measuring instruments for determining dioptres. Long-sighted people need plus lenses that focus the incident light rays in a focal point like a magnifying glass.
In contrast to short-sighted people, long-sighted people cannot assess their ametropia from their personal field of vision. Instead, the value for a lens is calculated for long-sighted people. The distance from the lens to the focal point is called the focal length.
The dioptre number of the plus lens is the reciprocal of the focal length. Examples: With a plus lens, the light rays meet in one meter, so the lens has the power plus 1 diopter. If they meet in 50 centimeters, the strength is plus two diopters.
If the focal point is 33 centimeters away, this means a dioptre of plus 3. The rule to be derived from this is: the shorter the focal length, the stronger the plus lens.
