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Basic question about contrast and gamma
"Monica Schulz" wrote in message ... Maybe 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 canīt 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 important 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 doesnīt 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/tut...hop-curves.htm. But we canīt 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 Iīd 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 flashing. 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 "portrait" 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 |
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