When I have this test, I’m told to leave my distance glasses on. If I’m struggling, the nurse gives me pinhole glasses which help focus the eye through tiny holes.
The Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, which may be one of several letters, for example E, H, or N.
Only ten letters are used C, D, E, F, L, N, O, P, T, Z.
Subsequent rows have increasing numbers of letters that decrease in size. A person taking the test covers one eye from 6 metres or 20 feet away, and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that specific eye.
One Vision Blog 