Want to learn more about Snellen eye charts? A good article from Lorrie Frear an Associate Professor in the School of Design/College of Imaging Arts and Sciences at the Rochester Institute of Technology in Rochester, New York.
My graphic design students love to design posters using the classic eye chart composition, and they frequently ask “What typeface should I use for this?” Not having a definitive answer has always been frustrating, so I decided to investigate to find out what typeface is used on eye charts.
What are Optotypes article
“As a result of continual refinements, most of today’s Snellen Charts follow logarithmic progression, have improved letter designs, and a uniform 25% progression from line to line.”
Article is from Graphics designer, John Boardley’s, I Love Typography web site.
And some interesting Snellen poster art ->
Islands of Math 