The diopter is a unit of measurement provided by the optic that indicates the refractive power of a lens.
The tenths, instead, indicate the distance from which a person can correctly visualize certain images. The degree of visual acuity in tenths is measured with the ototypic table (table used for eye exams with rows of letters or symbols of progressive size to simulate vision up to ten meters away). In practice, the so-called tenth indicates the number of lines of the optotype that the patient is able to read; an emmetropic eye (with normal refraction) can read up to 10/10 without lenses.
