I always say the librarian thing
My personal favorite, "you should get that checked out or something, you can't see anything'
thats why i have glasses
It's quite hard to NW this post, since those 4 things are pretty comprehensive...covers a huge majority of people
I always get, "Wow you're blind."
Or, "They make my eyes hurt."
Or "Why are you wearing sunglasses during the day?"
or "The floor is lava!"