Look at this interesting explainer on eye charts and human visual acuity

A Snellen chart (below) may just look like random letters in different sizes, but they were carefully designed to measure visual acuity. This piece on testing the limits of human vision brings it all into focus.

Fun fact: most ophthalmologists have switched to a LogMAR chart (below):

Testing the limits of human vision (YouTube / Verge Science)

Wikimedia images: LogMAR and Snellen