What is a diopter?

Hardly any other term is used so often by opticians – but hardly anyone knows exactly what dioptres mean. An attempt at an explanation: The diopter is a unit of measurement for the strength with which a lens breaks the light.  The diopter is therefore also a key figure for the ametropia of the eye. Minus values ​​correspond to  short-sightedness , plus values ​​to long-sightedness. Whether positive or negative: the higher the diopter number, the stronger the refractive power of the lens and thus the ametropia. At the optician’s, the refractive power of the lenses is almost always specified in quarter diopter steps (0.25 diopter steps). Finer gradations are only very rarely available.

Short-sighted and far-sighted

Without glasses , a short-sighted person can   only see clearly up close. From a maximum distance, everything becomes indistinct. Incidentally, with this maximum distance of sharp vision, the short-sighted can estimate the number of dioptres of the corrective lens themselves with some accuracy.

An example: If a myopic person can see clearly up to a maximum of one meter without his glasses, he needs a pair of glasses of minus one dioptre to see at a distance. With a visual acuity of up to 50 centimetres, it is already minus two dioptres, whoever sees clearly 33 centimeters needs a lens with three dioptres – and whoever is at minus eight dioptres can still see an eighth of a meter or 12.5 centimeters unclouded into the “distance ” see. These self-experiments are of course imprecise.

Exact measurements

Opticians have precise measuring devices to determine the diopter. Long-sighted people need plus lenses that focus incident light rays in a focal point, much like a magnifying glass. Unlike short-sighted people, far-sighted people cannot derive their ametropia from their personal field of focus.

Here you have to calculate: The distance from the lens to the focal point is called the focal length. The diopter number of the plus lens is equal to the reciprocal of the focal length. An example: If the light rays of a plus lens meet in one meter, the lens has the strength plus 1 diopter. If they meet within 50 centimetres, the strength is plus two dioptres. If the focal point is 33 centimeters away, the diopter number is 3. The rule applies: the shorter the focal length, the stronger the plus lens.