[pure-silver] Re: multigrade paper is amazing
- From: "Nicholas O. Lindan" <nolindan@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 8 Nov 2007 20:28:35 -0500
“Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
> Shannon Stone asked:
> How does VC paper work?
I have seen two explanations of how VC paper works. ... Ilford's
explanation is that there are two layers or components which are of the
same contrast but different color sensitization
Ilford, no surprise, knows what they are talking about.
They have a paper on how it works on their web site.
The technique was invented in the late 1800’s
(? - give or take a century or two).
Classicly, the paper is made with two identical
emulsions. When exposed separately each emulsion
has a DMax of 1.0 OD. When both are exposed
DMax is 2.0OD. [numbers are for example only]
When the intensity of exposure is reduced the
resulting HD curve is shifted to the right:
------- 1.0 OD
/
Normal 0.0----/
light ------- 1.0
/
Reduced 0.0---------/
light
Time ---->
Using this ability to shift, the high contrast
scenario is:
------- 1.0
Emulsion #1 /
Normal 0.0----/
------- 1.0
Emulsion #2 /
Normal 0.0----/
/------- 2.0
|
|
Combined |
Result 0.0----/
Time ---->
If one of the emulsion's exposure is attenuated the
result is low contrast.
------- 1.0
Emulsion #1 /
Normal 0.0----/
------- 1.0
Emulsion #2 /
Reduced 0.0--------/
------- 2.0
/
/
Combined /
Result 0.0-----/
Time ---->
The Pons Asinorum is the idea that when exposure is reduced
the HD curves shrinks downward. It doesn't - it shifts to
the right. It is easier to see the shift if the horizonal axis
is labled 'time'. The same curves ensue if time is held
constant and negative density is used to control exposure.
The examples above show a 2:1 contrast variation.
Ilford MGIV and the late Kodak Polymax use 3 emulsions -
green, cyan and blue - which, with some black magic,
achieve a 1:3 contrast range.
If you look at the paper’s curves you can see the bumps
in the contrast grades growing as the emulsion curves are
‘pulled apart’ to lower contrast. It looks that the
cyan emulsion is also used to smooth the response curves.
http://www.darkroomautomation.com/support/mgivfb-zone.jpg
==
Nicholas Lindan
Darkroom Automation
http://www.darkroomautomation.com
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