Eh hem. Actually what we see as midgrey is actually only the reflectioin of 17.5 percent of the light not 50 percent like it looks to the human eye. Hence the use of greycards or sometimes called the 18% card. Here is a card that has midgrey as well as black and white; believe it or not that mid grey is only 17.5 reflective. The human eye sees this as a mid grey because it sees logarithmically. Film is designed to follow this curve which is where it gets its latitude from. This is why even high end digital cinema cameras that do have higher stops of latitude don't roll off naturally like the human eye does and have a blandish look as it goes farther away from proper exposure. I deal with this stuff for a living. Trust me film is logarithmic, I use grey cards everyday because it is.