Probably no other term is used so often by opticians – but hardly anyone knows exactly what is meant by diopters. An attempt at an explanation: the diopter is a unit of measurement for the strength with which an eyeglass lens refracts light. Thus, the diopter is also an indicator for the refractive error of the eye. Minus values correspond to nearsightedness, plus values to farsightedness. Whether positive or negative: the higher the diopter number, the greater the refractive power of the lens and thus the defective vision. At the optician, the refractive power of the lenses is almost always given in quarter-diopter steps (0.25 diopter steps).Only very rarely are there finer gradations.
Short-sighted and long-sighted
A nearsighted person can see sharply without glasses only at near distances. Beyond a maximum distance, everything becomes blurry. By the way, with this maximum distance of sharp vision, short-sighted people can reasonably accurately estimate the diopter number of the correcting lens itself.
Example: If a nearsighted person can see clearly without his glasses up to a maximum of one meter, he needs a lens of minus one diopter to see in the distance. For visual acuity up to 50 centimeters, it is already minus two diopters, who sees 33 centimeters far sharply needs a lens with three diopters – and who is at minus eight diopters, can still see an eighth of a meter or 12.5 centimeters far into the “distance” unclouded. These self-experiments are, of course, inaccurate.
Exact measurements
Optometrists have accurate measuring devices to determine diopters. Farsighted people need plus lenses, which focus incoming light rays in a focal point much like a magnifying glass. Unlike nearsighted people, farsighted people cannot deduce their refractive error themselves from their personal acuity range.
Here it is a matter of 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. For example, if the light rays meet at a plus lens at one meter, the lens has a power plus 1 diopter. If they meet at 50 centimeters, the power is plus two diopters. If the focal point is 33 centimeters away, the diopter is 3. The rule is: the shorter the focal length, the stronger the plus lens.
