The vision can be divided in central and outlying vision. The central visual sharpness is measured being shown objects of different sizes at a distance standard of the eye. For instance, to relative " Table of Snellen ", it is composed of a progressive series of smaller arrays of aleatory letters used to measure the vision the distance.

Each array is designated by a number, corresponding the distance in the qualum normal eye is capable to read all the letters of the array. For instance, the letters in the array " 40 " are sufficiently big so that a normal eye sees in the distance of 40 feet. For convention, the vision can be measured or in the distance of 20 feet (6 meters), or still close, to 14 inches of distance. For diagnosis ends, the distance of the sharpness is the pattern for comparison, being always tested separately each eye. The sharpness is marked with two numbers (for instance, 20/40 "). The first number represents the test distance in feet between the picture and the patient, and the second represents the smaller array of the letters that the patient's eye can read. 20/20 are a normal vision; 20/60 indicate that the patient's eye can just read sufficiently letters big in a distance of 20 feet, which a normal eye can read in a distance of 60 foot.