I can change my vision from blurred to clear at will, I can not do it for an unlimited amount of time, but I bet if I spent more time doing it I could do it for long periods of time. At the moment I can do it for about 1-2 minutes tops before my vision goes back to clear. This doesn't hurt or take much effort. I until now had no idea that other people couldn't do it, I discovered I could do this at around the age of 6 accidentally, and I've been able to do it since.
Also GoddessDangelia I can do that myself my guess is that it's because humans have 2 eyes, lol. Try doing it with one eye closed.