They Didn't Wear Glasses and They were Probably Fine
TL/DR: They didn't wear glasses and all but the blindest of knights were at no more a disadvantage than a guy who was slightly shorter or weaker or slower than average.
As mentioned previously the kinds of glasses available to a medieval aristocrat were essentially impossible to use on a battlefield. But that's not a huge hindrance. A knight's ability too see clearly would not have a tremendously bad effect on his combat capability until you get in the range of near-blindness. You must consider the ranges at which the average knight was fighting. My own vision is atrocious, in the 20/400 range. Anything outside of about 20cm gets fuzzy, and I have difficulty identifying individuals at distances greater than 3 meters unless they're especially tall/short/weird hair color or the only person of a specific race in the room. 20/400 is "legally blind" without corrective lenses. But I can (and have) done ARMA-style fights without corrective lenses. I can still see motion at 3 meters, even slight movements. and I can also perceive objects like, say, a spear or a sword moving at that range. Even from 20ish meters away I can still identify things like "that's a person" and "that person is moving towards/away/sideways as about X speed" and can roughly distinguish the size of a weapon they're carrying. (I can tell if something a spear or a sword, but not really a spear or a halberd for instance) What does that mean for a knight with a similar impairment?
Honestly not a whole lot. Certainly a GENERAL might have problems, as he would not be able to easily identify troop formations or banners at a distance. But your average knight was a close-combat specialist fighting either on foot or mounted. On foot he's coming within a few feet of his opponent, where his vision is "good enough" to allow him to fight with confidence, especially against an armored opponent where subtleties like facial cues don't matter and fights often ended in grappling. Against mounted opponents he'd be at a slight disadvantage, but if his reflexes were good enough he could still get his lance's final aimpoint on target. As he's part of a larger formation somebody else will be able to point him at the enemy. Unlike the movies battles didn't devolve in wild mixed-in melees so as long as he stayed "pointed" the right direction so he didn't go running up to foes he thought were friends until he was 3 meters away he'd be ok!
At the end of the day it certainly is a bit more challenging to fight without good vision than with my contacts in. But I wouldn't rate it much more difficult than fighting a guy who has slightly longer reach or is slightly stronger or any of the other minor advantages that nature bestows on people. A knight isn't trying to see a thin flitting rapier, and catching arrows on your shield is a matter of luck and good positioning rather than seeing the arrow coming. A knight is trying to see fairly large objects at fairly close range, and at that point even the just-barely-legally-blind can do ok!