Digital BW, The Print

Martin writes:

> If the same 256 levels of gray pixels are fed
> to the printer and it has four channels each
> with a different shade of gray ink, it would
> seem to follow that each channel will print
> those 256 levels differently resulting in
> overall tonal interpolation. Lower than 1024
> but greater than 256.

If an image is printed at its original resolution, the number of gray levels
will never exceed the number present in the original.  So if the original
has 256 levels, the print will have 256 levels as well.

If an image is printed at a resolution different from the original, or if it
is manipulated in any way prior to printing as part of the printing process,
the printed version may have more gray levels than the original.  The
overall quality of the printed image is never improved by this process, but
it may enhance the visual impression created by the print.  For example, if
the original has only half the resolution of the printer, the printer or its
driver may interpolate the original to produce a printable version with
higher theoretical resolution.  This interpolation will typically involve
creating new pixels that may have gray levels intermediate to those present
in the original; so an original with 256 levels may be printed as an
interpolated image with 1000 gray levels or more.  However, this doesn't
improve the quality of the image; it simply makes pixellation less obvious
in the result.

There are no circumstances in which printing with more gray levels than the
original image contained results in a better printed image (in terms of
resolution or useful tonality).  You cannot create useful information that
was not already in the original image.  There are no exceptions to this
rule; it is a consequence of information theory.