In ophthalmology, the most common visual acuity chart used to test the eye’s visual function is the International Standard Visual Acuity Chart, which is most commonly seen as a chart consisting of 14 rows of “E” letters of different sizes and directions.
However, the number of degrees on the chart and the refractive error (myopia) are two separate concepts and there is no absolutely accurate conversion between them. A visual acuity chart measures visual acuity, which is the ability of the eye to distinguish fine structures, and does not reflect the refractive power of the eye.