The human eye is 50mm? Not quite ... as an amateur astronomer, this is what my various recent (and old) factbooks list and are considered to be the accepted focal length(s):
Object focal length of the eye = 17 mm
Image focal length of the eye = 22 mm
Dark adapted eye is about an ISO of 800
MP equivalent estimate is just under 600.
Here's just one of many places to check: http://hypertextbook.com/facts/2002/...retskaya.shtml
On edit, here's an astronomy reference, also: http://www.clarkvision.com/imagedeta...esolution.html