Digital BW, The Print

Some interesting links dealing with the number of graytones the human eye
can discern:

This one says 64:

http://orbit-net.nesdis.noaa.gov/arad/fpdt/tutorial/enhance.html

This one says 200 (and is a great primer on the halftone process):

http://www.adobe.com/support/techguides/printpublishing/scanning/psscanning0
2.html

And this one says between 30 and 65...but up to 80 on a monitor:

http://www.fortunecity.com/victorian/canterbury/222/astrovid6.htm

And this one even has publication references...and states 100:1:

http://www.njit.edu/Publications/Reports/HINT/CHAPTER4.html

And I quote:

"The sensation of lightness can distinguish luminance levels within about a
two-decade range, i.e. the eye is able to resolve a luminance variation (dL)
up to about 1/100 increment [Gonzalez and Woods, 1991, p. 34]. The luminance
discrimination threshold depends on the individual subject and the dark
adaptation of the eye during the test. Human vision adapts over a remarkably
large range of light intensity levels - about seven decades of dynamic range
in total [Billmeyer and Saltzman, 1981]. At each adaptation state the human
eye can discern a dynamic range as high as 100:1."

Austin