The human eye is an impressive organ, it is incredibly sensitive. In fact, it is theoretically possible to see a candle flame from a distance going up to 48 kilometers.
To see a candle flame from so far away, either one should be at the top of a mountain or that the ground should be flat during a dark night.
Our eyes can also see farther, since visual acuity depends on the number particles of photons (light) emitted by an object. The most distant visible element that can be seen by the naked eye is the Andromeda galaxy, which is 2.6 million light-years away from Earth.