Digital BW, The Print

Eric,

to add to what you said:-
most cameras and scanners do work like your first table of levels per
"zone" at the image capture stage :
> 
> zone        levels
> 1                2048
> 2                1024
> 3                  512
> 4                  256
> 5                  128
> 6                    64
> 7                    32
> 8                    16
> 9                      8
> 10                    4
> 11                    2
> 
However it would be wrong to think that a device which saves in jpeg
cannot (in principle) record below your "zone" 8 (I'll stick with your
definition).  Perhaps you were not saying it did, but someone new to
the subject could be confused.

> and for an 8 bit image
> 
> zone        levels
> 1                128
> 2                  64
> 3                  32
> 4                  16
> 5                    8
> 6                    4
> 7                    2
> 8                    1
> ...

Between the capture stage (CCD etc.) and the file save (or RAW
convert, or wherever the 8 bit file is created) there is a
"characteristic curve" applied (to use the old term which I like). 
This breaks the linear relationship between the binary numbers and the
light-level recorded on the CCD.

For example, in many point-and-shoot cameras the contrast is set so
high that they cannot even record 8 zones, only 5 or so.  While on the
other hand RAW conversion (or the few cameras with really good low
contrast settings) allows the range to be increased beyond 8 zones
(you'll find discussions about exactly how much over the previous
month or so, but no need to go there now).

Your main point is exactly correct: after the image is converted to 8
bits, any significant re-expansion of shadow contrast, or indeed any
sufficiently steep curve applied for whatever purpose, is going to
produce posterisation.  (Subtle posterisation can be visible in the 
mid-tones of black and white images.)

Ken




> 
> Obviously, with the 8 bit image, if you try to open up your shadows
 
> at all, they'll posterize.  Whereas the 16 bit image will have a  
> greater latitude for manipulation.
> 
> Well that is about the extent of my knowledge regarding this topic.
  
> An additional note however, is that this is the reason that you
want  
> to expose for your highlights and "develop" for the shadows with  
> digital, the exact opposite of how one approaches exposure with
film.
> 
> Hope that helps.
> 
> Eric
> www.ericashworth.net