How far can the human eye see? Any finite distance or infinite? How far can the naked human eye see if unobstructed? (straight ahead on perfectly flat land). Is there a stopping point or is it infinite when you look in the sky? Is there any situation that the human eye can see infinite?
What are the factors that determines how far we can see? The photon flux density and...?
Physics and biological point of views will both be interesting to discuss.
 A: *

*due to the curvature of the earth, you can only see flat land for about 5km give or take a few hundred meters.

*from a sky object you are limited by the photon flux emitted or reflected by that object (50-150 photons in a 1ms burst, and all landing within a 10 arcminute spot on your eye)

*the wavelength of those photons affect your sensitivity (green photons are best)

*the duration your eyes have spent in darkness (eye sensitivity increases 100 fold by 5th minute in darkness, and 1000x by 15th minute)
apart from 1., all the above information can be found in this famous paper: http://arapaho.nsuok.edu/~salmonto/vs2_lectures/Lecture6.pdf
A: We can see objects much more distant than the Andromeda Galaxy with the naked eye if they have unusually high intrinsic brightness (supernovae, gamma-ray bursts, etc.)
According to the following link, the current record is estimated to be 7.5 billion light years away: https://www.space.com/5153-stellar-explosion-distant-object-visible-naked-eye.html
