Actually, there is a real life eye chart that only uses the letter E. They messed up the appearance a bit here, but you notice how the top letter appears to be rotated? That’s indicative of a “Tumbling E” eye chart, which is used to detect nearsightedness and visual acuity in very young children (who are unfamiliar with all the letters of the alphabet), or children and adults who cannot communicate verbally (due to a disability, language barrier, or various other reasons).