Final answer:
The Snellen chart is the most common tool for measuring visual acuity, presenting letters of decreasing size to assess sharpness of focus at various distances.
Step-by-step explanation:
The most common test for measuring visual acuity is the Snellen chart. This chart features a series of lines with Roman letters in decreasing size. A person with normal vision should be able to read the line of letters that subtend an angle of 5 minutes of an arc from a distance of 20 feet. The different sizes of letters on the Snellen chart correspond to what a person with normal visual acuity should be able to read from various distances. For instance, the letter size representing 20/200 vision would be larger because someone with normal visual acuity should be able to read it from 200 feet away. Assessing visual acuity with the Snellen chart provides an estimation of one's sharpness of focus.