It's called
Amblyopia, and it effects some 3-6% of the population.
Basically, you do not see in stereoscopic vision because one eye is blurry. Your brain learns to block the blurry image, and you only end up using one eye.
The vision in the unused eye deteriorates until functional blindness.
I'm probably something like 20/100 in my right/left eyes, respectivly.
🙁 I don't know, I can't even read the text on the screen a foot away from me with my left eye. It's actually quite alarming, since I can remember when I used to be able to see out of both eyes fine.
It also messes with your depth perception, since you don't have stereo vision. My brain has learned to compensate though, and I'll almost never miss on a depth-judgement.
Fortunately, you can really only tell when I'm tired. It's an eye muscle thing, I guess. Sometimes it is more apparent than others, kinda weird really. It used to be a lot worse, but i've had some 4 surgeries to correct my left eye's muscles.