The Middle Ages was a very inventive period, and one of the things they invented was eyeglasses.
Many people have always been nearsighted or farsighted, but until glasses were invented there was nothing to do be done about it. A lot of medieval manuscripts look as if written by a nearsighted person writing with his nose very close to the parchment--the letters are tiny, sometimes hard to decipher without a magnifying glass (or a modern nearsighted person taking off her glasses). Alternately, a lot of medieval people must have been farsighted, to make out details in high stained glass windows that require most of us to use binoculars.
Medieval people worked with glass especially for church windows, but also for decorative purposes and as a stand-in for jewels. It doesn't take much to discover that glass can bend light, to focus it at different spots if it's convex or concave. The Arabs had written on optics, giving ideas to thirteenth-century scholars. The first known depiction of a person wearing glasses dates from the end of the thirteenth century. During the fourteenth century, eyeglasses became relatively common among the well-to-do.
The above image dates to around 1400.
(Bifocals, by the way, were invented by Benjamin Franklin in the eighteenth century. Until then, someone might need two different pairs of glasses.)
We now take glasses so much for granted--not to mention contacts and laser surgery to correct sight--that we may not appreciate what a useful invention they were. Without them, a lot of people would have wandered half-blind through their lives.
Including me. I would have been hopeless in the twelfth century.
Here's a safety tip. If you ever see what appears to be a sort of green curtain across part of your vision, it means your retina is coming detached from the back of your eye. Do not hesitate. Do not let that ooky feeling of thinking about eyeballs and sharp instruments in the same sentence deter you. It won't go away by itself. Diet, exercise, and herbal supplements will have no effect. In a few days the green curtain will be replaced by the black curtain, and then you're blind in that eye. Don't let that happen to you. For one thing, you'll be asleep during the operation and will miss the sharp instrument-meets-eyeball moment.
(If you're concerned, and I appreciate your concern, mine was successfully caught at the green-curtain stage.)