<P> where w (\ displaystyle w) is the optotype height or width (which are the same due to the optotype being on a square grid), d (\ displaystyle d) is the distance from eye to chart, and θ (\ displaystyle \ theta) is the angle subtended by the optotype, which is 5 arcminutes as specified by Snellen . Another calculation for United States clinics using 20 foot chart distances (slightly more than 6m), and using a 17mm model eye for calculations, and a letter which subtends 5 minutes of arc, gives a vertical height of the 20 / 20 letter to be 8.75 mm . </P> <P> Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription . During such examinations, acuity ratios are never mentioned . </P> <P> The largest letter on an eye chart often represents an acuity of 6 / 60 (20 / 200), the value that is considered "legally blind" in the US . Some individuals with moderate myopia may not be able to read the large E without glasses, but have no problem reading the 6 / 6.0 (20 / 20) line or 6 / 4.5 (20 / 15) line with glasses . By contrast, legally blind individuals have a visual acuity of 6 / 60 (20 / 200) or less when using the best corrective lens . </P>

What letter is at the top of the snellen chart