Outside of injuries, ailments or other maladies that would leave one sight-impaired to some degree, when did people become aware that eyesight didn't have a "universal" quality?
Would a medieval peasant with myopia assume that everyone saw a blurry world, just like them? Would scouts on the Great Wall be assigned to posts based on their comparatively keen eyesight?
You're not going to get an in-depth direct answer to your first question because this would have been obvious long before writing and thus history began. Vision often deteriorates slowly as we age, giving first hand knowledge to many people that useable uninjured vision varies. It's also easy to test whether two people can both make out some detail of a distant object and thus directly compare eyesight for when a task calls for the person with the more acute vision.
For your second question I hope you can learn something from u/sunagainstgold's excellent answer to a question similar to your second asked previously.