Re: Basic question about contrast and gamma

"Monica Schulz" <monica.schulz@[EMAIL PROTECTED]
> wrote in message 
news:1c545b43-672d-4790-9803-c00e025cb69a@[EMAIL PROTECTED]
 I did not make myself clear enough in my last post. 
What I mean
is that there is no need for such a wide variaty of paper 
grades in
color printing as there is in b&w because contrast can´t be
manipulated that much in color as in b&w. I came thinking 
about that
after reading another old thread so I will use the im****tant 
words of
the relevant post.
To adjust contrast means to make the image brighter or 
darker. The
only way to do that is to make the dyes thinner. This lets 
more light
reflect off the paper. But as a dye gets, say, thinner it 
removes less
of its anti-color until it finally disappears and no 
anti-color is
removed. And the other way around if the dye gets thicker. 
So the
scale doesn´t go from, say, a dark magenta to a light 
magenta but from
dark magenta to white. Or from a saturated magenta to an 
unsaturated
magenta.
The digital process can compensate for this unwanted 
increase or
decrease in saturation if contrast is adjusted in the 
luminosity
channel of lab-mode. In this case neither hue nor saturation 
is beeing
changed. There is a nice little example of that on
http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm.

But
we can´t do the same in analog color printing.
It could well be that this is complete garbage as far as the
subtractive mixing of the analog process is concerned and if 
so I´d be
happy if someone tells me so that I can get this idea out of 
my head
and can think in another direction.

Best regards!
Monica Schulz


     Something very like the "curves" effect can be done in 
chemical photography be means of elaborate masking and by 
using such techniques as coltrolled fla****ng. Both 
techniques were widely used in making the plates for 
four-color printing. However, its very much easier to do it 
in Photoshop and one can see immediately the results. Most 
of the tailoring done in Photoshop can be done in analogue 
photography but can be very complex to do there.
     Contrast and brightness are different. This is 
especially true of most reproduction media where the maximum 
brightness is determined by illumination. What adjusting 
contrast does is to fix the points that are the minimum and 
maximum brightness of the image. As contrast is increased 
something must be lost. That is where the "brightness" 
adjustment comes in. That controlls what is to be the 
midpoint of the gray scale. Its possible to extend the range 
of tones that will reproduce without falling off one end or 
the other by shaping the transfer curve and that is what 
Photoshop is doing. Many films and papers have curves which 
affect the reproduction of gray tones because they are not 
straight lines. Similar curves are available in color films 
and papers. This one of the things that determines the 
difference between "****trait" material and commercial 
material. The overall conrast may be the same but the 
placement of reproduced tones in relation to the originals 
is different.


-- 
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]