"The human eye is so incredibly sensitive that it can actually detect a single particle of light – a photon.
"The
truth is, the human eye is so sensitive that were it not for special
features inside the eye that process the billions of pieces of
information coming into the eye every split second, we would be
overwhelmed. While the eye can detect even one photon of light, it will
not pass an image on to your brain until at least six photons strike in
the same area of the eye. If this weren’t the case, on a dark night we
would see nothing but static, since less than six photons could not be
focused into an image and would appear to us as just static.
"The range
of the eye’s sensitivity is also a million times greater than our
modern photographic films, providing us with a dynamic range of 10
billion to one. While the greatest sensitivity is needed on dark nights,
an internal control in the eye reduces that sensitivity for bright
daylight."
