Fig. 13. This chart depicts a selection of commonly used optotypes. Top: Snellen H (with serifs, 5 × 5), Sloan H (no serifs, 5 × 5), British H (no serifs, 4 × 5). Bottom: LEA symbol, tumbling E, Landolt C. The latter groups have only four symbols each so that the guessing level is higher than with letter charts. Most optotypes approximate the recognizability of Landolt Cs; they represent 20/20 (1.0) acuity when their height subtends 5 minutes. Recognition within the LEA symbol set is more difficult; calibration experiments established that the symbols needed to be about 35% larger.