Sharp is not the whole story
Small detail and low contrast are different demands on vision. Change one at a time to see why a letter chart cannot describe every part of useful sight.
Can something be sharp and still hard to make out?
Try the idea
Sharp edges only help when they stand out from the background. Contrast, print size and blur each change how easy a task is.
Change print size, contrast and blur one at a time. Which change makes the appointment card easier to read?
Turn on JavaScript to use the interactive experiment. The written introduction and the original source stay available here.
RNIB: low-vision services