Estimation of the strength of a visual aid for
If there is an age farsightedness, then there is a rule of thumb, which helps to the inaccurate estimate: The reciprocal value of the distance in meters, in which one would like to read its newspaper gladly becomes minus the reciprocal value of the distance in meters counted, in which one can see still sharply. This results in the refractive power of the reading glasses. As an example the following shall be mentioned: The desired reading distance is 1/3 m and therefore the reciprocal value is 3 diopters.
The smallest visual distance at which sharp vision is still possible is 1/2 meter and therefore 2 diopters. Therefore 3 dpt – 2 dpt = 1 dpt must be calculated. Thus, reading glasses with a power of +1 dpt are necessary to be able to read well at a distance of 1/3 m. The value of the required diopter can be calculated more reliably by an eye test at the ophthalmologist or optician.
The so-called Landolt rings are often used for this purpose. They owe their name to the Swiss ophthalmologist Edmund Landolt, who lived in Switzerland and France from 1846 to 1926. These Landolt rings have an opening on one side that is difficult to detect. If it is still possible to see this opening on the ring at an angle of vision of only one minute of arc (e.g. 1.45 mm from a distance of 5 m), the visual acuity is 1.0 and one is therefore 100% normal sighted.
