Digital BW, The Print

A little bit of computer science here.

An 8 bit BW file can have at most 256 levels of gray tones, a 16 bit
file can have 65536 independent tones. Converting from 8 to 16 bit
simply maps those original 256 tones into the 65536 space. If you had
a linear gradient going from black to white at the start it would
appear exactly the same after the conversion. You would still have 256
independent tones. However you would also have room between each tone,
256 blank spots with no gray vales on either side. By this I mean that
for the gray value that was originally 128 in the 8 bit file it would
now be 32768. The difference being that in the 8 bit file there were
picture elements represented by values 127 and 129. While in the 16
bit file those values would be represented by 32512 and 330224, there
would be no pixels in the image with values between 32768 and these
other two numbers.

Next lets say that you adjust the global contrast of your image. In a
simplified algorithm, an increase in contrast makes the values below
the midpoint (128 8-bit) darker and the values above the midpoint
lighter. In an 8 bit file if we subtract 1 from each value less than
128 then we now have 126 tones where we had 127 tones before the
subtraction. This is because the pixels that were formerly at value 1
are now at zero along with all the pixels that were previously at
zero. The same logic applies above the midpoint, so where we
originally had 256 tones before the contrast adjustment we now have
254. In a 16 bit file applying the same type of correction still
leaves you with the original 256 independent tones, because there was
256 bits of headroom between each original pixel value. You would have
to adjust the contrast by more than 255 points before you started
reducing the number of tones in your image.

The scenario I've described above leads to the posterization that we
see when making large adjustments in 8-bit files. It is not true that
all image editing leads to the loss of data, but large movements
certainly can. There are other issues with 8 vs 16 bit related to
rounding errors also.

With all that said, I rarely change my 8-bit files to 16-bit before
editing. I would only do so if there was something really wrong with
my exposure and I needed to make large adjustments to the file.

Hope I didn't confuse the issue too much.

Derek