There are no historic eye exam records from the middle ages, I'm afraid. There are anecdotal examples, however: We can't know to what degree they were visually impaired, but a fair number of Venetian Doges managed to get a remarkable amount of things done even though they were described as "Blind" by their contemporaries, so clearly the actual outcome of a visual impairment depends on the resources available to the person in question. A farmer or bricklayer can continue to work even if if slightly nearsighted, and a person with attendants, staff, and a support network can continue to function well into near-blindness. Lastly, I gave a brief answer to a similar question about the Doge Enrico Dandolo about two weeks ago you might be interested in.