How far can the human eye see? — Rick, Santa Ana, Calif.
If photons hit the human eye, the human eye can see them. So if there is a large galaxy a million light years away, and if it produces enough photons to reach your eye, then it is possible for you to see something that is a million light years away. The Andromeda galaxy is 2.25 million light years away, and most people can see it with the naked eye. If you add a telescope, you can see even farther.
If there is a supernova or gamma ray burst, it can create a gigantic number of photons for a short period of time. This article discusses a gamma ray burst (caused by a collapsing star) 7.5 billion light years away that was visible to the naked eye for a short period of time. This appears to be the farthest object that the human eye has been able to see.
More info on the Andromeda galaxy: