U of S | Mailing List Archive | alt-photo-process-l | Lost of contrast

Lost of contrast



Hi all,

An ideal process and paper would scan the paper value as 255 and its darkest
value as 0. Unfortunatly, this ideal process doesn't exist. In order to deal
with this, the linearisation procedure was introduce, basically it is
suggested by most people here to expand the process print range to those
above value (0, 255) and using the curve tool, you manage to obtain a one to
one match for intermediate values. This must satisfy most of you, otherwise
the contrast problem would have been raised a long time ago. This procedure
is called linearisation by most of you here, in the color management world
it is called gamut or tone mapping especially in the case of monochrome
images.

The problem I raise is though this linearisation procedure is easy to do in
practice and may pretty well give you acceptable results or prints it as the
adverse effect of reducing the overall contrast of your work. In order to
find a one to one match between the original and the print you have chosen
to expand the range of the print and this as the effect of increasing the
print contrast right? Suppose now that you would have chosen to compress the
original range to print range, this of course would have reduce the contrast
of the original and it would still have allow you to obtain a one to one
match again. That's one of the fun things of working with numbers, one can
approach a problem different ways and obtain equivalent result.

Now lets look at the real thing and compare the original values ranging from
0 to 255 to the actual printed values. The maximum white of the original as
been translated to some value lower then 255 and the darkest original value
(0) as been translated to some value above 0. It would seem to me, this is
equivalent to a compression of the original range to the print range. As
explained above, we know that compressing the range of an image (in a linear
fashion) causes a reduction in contrast.

As I said from the start, you can be satisfied with this effective lost of
contrast and keep doing this linearisation procedure as you've doing for
eons. Knowing this though, I for one would certainly try to find a better
way and I actually found 1 and created another that is just as easy to
implement in practice as this linearisation scheme you all seem to use but
without loosing as much contrast.

Have a nice day to you all,
Yves