Someone should sticky the previous thread that had a long biological explaination on what our eye can and cannot see. And how 50mm actually has no real relation to our biological makeup.
The '50mm' approximation comes from the fact that 35mm film (aka 'full-frame') has a diagonal of 43mm, and a lens with that focal length would produce an image of least distortion, also popularly (ie conveniently, albeit erroneously) known as 'what the eye sees'.
The concept transfers itself to any format. For 1.6x crop, the diagonal of the sensor is ~27mm, and a 28mm lens would provide 'closest focal length of the human eye'.
HTH.
p/s to get the diagonal of any format, use pythagoras theorem on the sensor's/film's length and width.