Hardly any other term is used so often in the optometrist - but hardly anyone knows exactly what is meant by diopters. The attempt to explain: Diopter is a measure of the strength with which a lens breaks the light. Thus, the diopter is also a measure of the refractive error of the eye. Minus values correspond to myopia, plus values of farsightedness. Whether positive or negative: the higher the dioptric number, the stronger the refractive power of the spectacle lens and thus the refractive error. In the optician, the refractive power of the lenses is almost always in quarter-diopter levels (0.25 diopter levels) specified. Only very rarely there are finer gradations.

## Short and farsighted

A nearsighted person can only see things close without glasses. From a maximum distance everything becomes indistinct. Incidentally, with this maximum distance of acuity visionaries can estimate the dioptric number of the corrective ophthalmic lens fairly accurately.

For example, if a short-sighted person can see clearly up to a meter without his or her glasses, he needs a lens of minus one dioptric to see in the distance. With the visual acuity to 50 centimeters it is already minus two dioptres, who sees 33 centimeters far sharp needs a spectacle lens with three dioptres - and who is at minus eight dioptres, can still an eighth or 12.5 centimeters wide unclouded in the "distant "see. These self-experiments are of course inaccurate.

## Exact measurements

Optometrists have precise measuring devices for dioptre determination. Far-sighted people need Plus lenses that focus light rays at a focal point much like a magnifying glass. Unlike shortsighted people, farsighted people can not derive their ametropia from their own personal focus area.

Calculation: 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 beams meet with a plus glass in one meter, the lens has the strength plus 1 diopter. If they meet in 50 centimeters, the strength is plus two diopters. If the focal point is 33 centimeters away, the diopter number is 3. The rule is: The shorter the focal length, the stronger the plus glass.